ENH: unit of measurement / physical quantities
quantities related
xref #2494 
xref #1071 

custom meta-data
xref #2485 

It would be very convenient if unit support could be integrated into pandas.
Idea: pandas checks for the presence of a unit-attribute of columns and - if present - uses it
- with 'print' to show the units e.g. below the column names
- to calculate 'under the hood' with these units similar to the example below

For my example I use the module pint and add an attribute 'unit' to columns (and a 'title'...). 

Example:

``` python
from pandas import DataFrame as DF
from pint import UnitRegistry
units = UnitRegistry()

class ColumnDescription():
    '''Column description with additional attributes.

    The idea is to use this description to be able to add unit and title
    attributes to a column description in one step.

    A list of ColumnDescriptions is than used as argument to DataFrame()
    with unit support.
    '''

    def __init__(self, name, data, title = None, unit = None):
        '''
        Args:
            name (str): Name of the column..
            data (list): List of the column data.
            title (str): Title of the column. Defaults to None.
            unit (str): Unit of the column (see documentation of module pint).
                Defaults to None.

        '''

        self.data = data 
        '''(list): List of the column data.'''

        self.name = name
        '''(str): Name of the column, naming convention similar to python variables.

        Used to access the column with pandas syntax, e.g. df['column'] or df.column.
        '''

        self.title = title 
        '''(str): Title of the column. 

        More human readable than the 'name'. E.g.:
        Title: 'This is a column title'.
        name: 'column_title'.
        '''

        self.unit = unit
        '''Unit of the column (see module pint).

        Intended to be used in calculations involving different columns.
        '''

class DataFrame(DF):
    '''Data Frame with support for ColumnDescriptions (e.g. unit support).

    1. See documentation of pandas.DataFrame.
    2. When used with ColumnDescriptions supports additional column attributes
    like title and unit.
    '''

    def __init__(self, data, title = None):
        '''
        Args:
            data (list or dict):
                1. Dict, as in documentation of DataFrame
                2. List of the column data (of type ColumnDescription).
            title (str): Title of the data frame. Defaults to None.
        '''

        if isinstance(data, list):
            if isinstance(data[0], ColumnDescription):
                d = {}

                for column in data:
                    d[column.name] = column.data

                super(DataFrame, self).__init__(d)

                for column in data:
                    self[column.name].title = column.title
                    self[column.name].unit = column.unit

                self.title = title

        else:
            super(DataFrame, self).__init__(data)

if __name__ == '__main__':

    data = [ ColumnDescription('length',
                               [1, 10],
                               title = 'Length in meter',
                               unit = 'meter'),
             ColumnDescription('time',
                               [10, 1],
                               title = 'Time in s',
                               unit = 's') ]

    d = {'length':[1, 10],
         'time': [10, 1]}
    df = DataFrame(d)
    print 'standard df'
    print df

    df = DataFrame(data)
    print '\n' + 'new df'
    print df

    ####use of dimensions####
    # pint works with numpy arrays
    # df[name] is currently not working with pint, but would be I think 
    # it would be a real enhancement if it would...
    test = df.as_matrix(['length']) * units(df['length'].unit) / \
           (df.as_matrix(['time']) * units(df['time'].unit))
    print '\n' + 'unit test'
    print test
    print '\n' + 'magnitude'
    print test.magnitude
    print '\n' + 'dimensionality'
    print test.dimensionality

```
