__repr__ method in Table now also returns the units

updated doctests (for __repr__ table)

updated doctests (for __repr__ table), corrected

modifying table_repr_method

show units only when atleast one column is not None. + doctests updated.

show units only when atleast one column not None. + doctests updated.

removed quotes from 'None'.

some code standardization

added test for repr method, updated changes.rst

added doctest for repr method, updated chages.rst
Merge pull request #2180 from rastogitanuj/table_repr_method

__repr__ method in Table now also returns the units