Add count_atoms to GrammaticalExpression

[Ashvin-Ranjan]

• This pull request will add the method count_atoms to the class GrammaticalExpression, as suggested by GrammaticalExpression _len_ does not return number of atoms / leaf nodes #42.
• This also adds the test test_atom_count to test_grammar.py.

Notes

• When implementing the test in test_grammar.py, numerous issues with testing came up
  • test_grammar.py and test_language.py both have issues with Universe expecting a tuple of priors
  • test_language.py also has some issues with a Meaning being initialized incorrectly
  • test_meaning in test_grammar.py fails (even after changes are reverted)
  • However after implementing temporary workarounds, test_atom_count does pass
• As mentioned in GrammaticalExpression _len_ does not return number of atoms / leaf nodes #42. count_atoms can also be moved to a helper method in grammar.py, I simply felt it was more in line with the current project structure to make it a method of the GrammaticalExpression class

[shanest]

Thanks, Ash! Much appreciated. One small change requested, just for readability :)

Re your point about testing: the tests are indeed quite outdated (we made some breaking changes to (at least) Universe and Meaning since they were written). If you're interested in digging more into ULTK, going through and cleaning up the test suite would be very welcome!

[shanest]

Nice! This can be streamlined slightly: after the original if statement on line 177, can just do return sum(child.count_atoms() for child in self.children) :)

[Ashvin-Ranjan]

Oops! I copied the code over from __len__ without realizing that it was a if statement, not a for loop, I have pushed a correction. Thank you for pointing it out.

[Ashvin-Ranjan]

Thank you for looking over the PR! I have implemented the suggestion you have made.

Regarding unit tests, I would be more than happy to look into them and clean them up as a way to familiarize myself more with ULTK as a whole.

[shanest]

Awesome; thanks for this and for taking a look at the tests next!