Set `Datac.ordinates` as array

If `calc_method` returns a value of type `astropy.units.Quantity`, the
plotting functionality of the `Datac` module will fail. This commit
adds some non-public functionality to ensure that `Datac.ordinates` is
set as an array-like `astropy.units.Quantity` in this case.
Test for issue #19

Closes #19.
Bump version number