Add tests for Entity methods

[Seth-Rothschild]

This PR adds three tests and fixes the add_variable method for Entity.

• Fixes a faulty assertion and expected column types in entity.add_variable
• Test entity.add_variable
• Adds tests for two value errors in entity.update_data
• Test the previously untested elif unit == 'Y' block in add_td

[codecov-io]

Codecov Report

Merging #262 into master will increase coverage by 0.29%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #262      +/-   ##
==========================================
+ Coverage   94.14%   94.43%   +0.29%     
==========================================
  Files          71       71              
  Lines        7632     7639       +7     
==========================================
+ Hits         7185     7214      +29     
+ Misses        447      425      -22

Impacted Files	Coverage Δ
featuretools/entityset/entity.py	93.49% <ø> (+3.71%)	⬆️
featuretools/tests/entityset_tests/test_entity.py	100% <100%> (ø)	⬆️
...aturetools/tests/entityset_tests/test_timedelta.py	100% <100%> (ø)	⬆️
featuretools/entityset/timedelta.py	86.42% <0%> (+5.71%)	⬆️

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[kmax12]

rather than add a test for add_variable, let's just remove the function. it's only used in one place internally and we can probably avoid needing it at all

[Seth-Rothschild]

It seems it's not possible to satisfy the conditional

if self.index is not None and self.index not in inferred_variable_types

which used add_variables. Either you set the index as the index of the entity and self.infer_variable_types will catch it or you set the variable type as variable_types.Index, in which case it will be automatically included in the inferred_variable_types dictionary.

If there is a situation which can happen, it would be easy to add the relevant lines from add_variable under this conditional.

[kmax12]

@Seth-Rothschild ya, that makes sense to me. let's remove that code block and the add_variables function.

edit: looks like you already did.

[kmax12]

Looks good. Merging