'add a double table, 2 tables with one column merged horizontally
Parameters
res : results instance
some required information is directly taken from the result
instance
title : string or None
if None, then a default title is used.
gleft : list of tuples
elements for the left table, tuples are (name, value) pairs
If gleft is None, then a default table is created
gright : list of tuples or None
elements for the right table, tuples are (name, value) pairs
yname : string or None
optional name for the endogenous variable, default is "y"
xname : list of strings or None
optional names for the exogenous variables, default is "var_xx"
Returns
None : tables are attached'

def add_table_2cols(self, res, title=None, gleft=None, gright=None, yname=None, xname=None):

    table = summary_top(res, title=title, gleft=gleft, gright=gright, yname=yname, xname=xname)
    self.tables.append(table)
