Allow nested brackets

[sjd210]

Allowing nested brackets is deceptively complicated compared to allowing single brackets (and only applies to the Chemistry side of the checker).

.bracketed is now a number value representing the number of brackets an Element/Compound/Ion is contained within, such that 0 or undefined represents anything unbracketed. (This is set in the pre-processing augmentNode stage, where any child copies the bracketed of its parent, except a Bracket which increments it by one).

bracketAtomCount is now an array of atom-counting Records of length equal to .bracketed for the node (operating like a stack). Elements are added to the top-level bracketAtomCount when encountered, and the top level count is added to the second level, multiplied by the subscript .coeff (then popped off) when a Bracket is closed.

termAtomCount functions similarly to bracketAtomCount but as the designated always-present bottom level of the stack. Unbracketed Elements are added directly to it, and it is multiplied by the Term's leading coefficient and added to atomCount directly when a Term is closed.

atomCount has not changed. It is only ever incremented when a Term is closed, and has a Fraction value due to our use of Fraction coefficients.

The same process applies to bracketChargeCount, termChargeCount, chargeCount - except that charge is only ever added at an Ion node rather than at an Element node.

The Nuclear Checker has a much more basic structure with just termNucleonCount and nucleonCount in the structure [atomCount, massCount].

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Also adds isChargeBalanced and sameCharge to the response, in order to allow the API to give feedback on them.

While making this change, more comments were added across the Checker to (hopefully) make it a lot clearer, as well as some restructuring work - mostly such that Statements have a separate CheckerResponse on their left and right side to stop cross-interference. They are merged for the final response that is logged/sent to API.

[sjd210]

The tests are failing on this branch, but this is an issue with the tests - not with the checker itself.

An in-progress branch with this nested bracket change and all tests working can be found here.