Consider removing/modifying LetterWithSubscripts

[bryano]

The current implementation seems pretty hacky and brittle.

For one, LetterWithSubscripts('x', 1, 2) == LetterWithSubscripts('x_1_2') == LetterWithSubscripts('x', '1_2'). According to @Strilanc, this is necessary because of how poorly Sympy treats subclasses of sympy.Symbol.

One possible alternative replace the class entirely with a function letter_with_subscripts that returns a sympy.Symbol object directly with the right name. The function could set the letter and subscripts attributes of the object, but that might not play well with mypy.

[bryano]

If it's left as is, the inputs should at least be validated or parsed so that LetterWithSubscripts('x_1_2') either throws an error or evaluates to LetterWithSubscripts('x', 1, 2).