Add tests for missing data

[ima-hima]

Make sure that continuous, additive, non-additive, sankoff, and dynamic characters work with both missing data and all data missing.

[recursion-ninja]

You marked it as a defect, does it not work as expected?

[ima-hima]

There wasn't a verification or testing tag.

[recursion-ninja]

Add integration test that include missing data for:

• [X] continuous
• [X] non-additive
• [X] additive (defective, see #90)
• [x] sankoff
• [x] dynamic (both missing elements in the sequence and missing sequences)

[recursion-ninja]

Added missing data test cases.