LightCurveCollection.stitch() should ignore unmergeable columns (fixes #954)

[barentsen]

This PR addresses #954.

The following snippet of code currently raises an exception because column 'x' is a QColumn in lc1 and a Quantity in lc2:

import lightkurve as lk
from astropy import units as u
lc1 = lk.LightCurve(data={'time': [1,2,3], 'x': [1,2,3]})
lc2 = lk.LightCurve(data={'time': [1,2,3], 'x': [1,2,3]*u.electron/u.second})
lk.LightCurveCollection([lc1, lc2]).stitch()

This PR changes the behavior to exclude column 'x' from the stitch and raise a warning about it.