rewrite expression parser/evaluator

[ChristopherChudzicki]

Rewrote much of calc.py and changed name to expressions.py

Noteable changes:

1. I tried to separate parsing and evaluation as much as possible. Usage now looks like:

   parser = MathParser()
   expr_1 = parser.parse('x + y')
   expr_2 = parser.parse('x*y')
   
   variables_A = { 'x': 5, 'y': 7 }
   variables_B = {'x': -3, 'y': 4}
   
   expr_1.eval(variables_A, {}, {}, {})
   expr_1.eval(variables_B, {}, {}, {}) # evaluate the same expression with different variables, do not re-parse
   
   expr_2.eval(variables_A, {}, {}, {})
   expr_2.eval(variables_B, {}, {}, {}) # evaluate the same expression with different variables, do not re-parse

2. I changed the grammar so that the parser accepts all alphabetic suffixes and '%'. Advantages:
   • Allows us to give better error messages for things like 'x + 5y'. We used to just say "Unable to parse". Now the expression DOES parse, but we can say "y not allowed directly after number, did you forget *?"
   • Resolves the weirdness in #132

[ChristopherChudzicki]

@jolyonb This is ready for review.

I had to specify pytest-cov version in requirements.txt. Newest version (2.6) dropped support for Python <3.4. We have the most recent version that supports Python 2.7.

https://pytest-cov.readthedocs.io/en/latest/changelog.html#id1

[jolyonb]

Feel free to mark any unconstructive comments as resolved, btw.

[ChristopherChudzicki]

I believe I've just resolved everything except for the adjacent issue. Just confirm you want that and I can change it, or merge as is.

[jolyonb]

If we don't break any tests by setting adjacent to True, then let's go ahead and do it.

[jolyonb]

Awesome job. This almost feels like it's worthy of a revision number!