ENH: Canonic SI frequency
Since pandas is widely used for data science, including working with time series, a canonic representation of the frequency, as defined by SI standards, physics etc. would be a nice improvement.

To summarize differences between the general definition of frequency and the pandas definition of frequency:
- General definition: Frequency is in the unit of `1/time_period`, for SI unis more specifically `1/second=1Hz`.
- pandas definition: Frequency is the time period, so it is exactly the opposite of the physical/general definition. 

Since many calculations require the actual frequency, for example summing up velocities to get the length, this would facilitate many calculations.

#### Proposed solution:
Additionally `pd.DatetimeIndex` etc. should get a new attribute, f.i. named `freq_canonic`, that represents the actual frequency of the index in Hertz.

To avoid breaking the API, I'd implement the frequency as a data attribute, which can be accessed by the user, but has no effect on time series calculations.

I could try making a pull request for this feature, if you are interested.