Reorganize variable name parsing

[ChristopherChudzicki]

This is a rewrite of the variable name parsing to address #25

One of two forms is acceptable:

1. front + subscripts + tail
2. front + lower_indices + upper_indices + tail

is allowed, where:

• front (required): Alphabetical followed by alphanumeric, e.g., Xab
• subscripts (optional): Alphanumeric and underscores, e.g., _cd___2_3
• lower_indices (optional): Like _{12xy3}
• upper_indices (optional): Like ^{4abc}
• tail (optional): Any number of primes.

So for example: XY__ab__c_ is allowed (front + subscripts) and ST_{ij34}^{bc} is allowed (front+lower_indices+upper_indices) but T_ijk^{123} is not allowed (front+subscriots+upper_indiices).

Using the language above, edX original is front+subscripts

[coveralls]

Coverage increased (+0.0002%) to 99.9% when pulling da7b08fc2bc60f913e6d87c821e35c0ff41e1b32 on fix-subscripts into 0326f14a43f8167addccd7eebdb794724de8b754 on master.

[jolyonb]

That looks reasonable to me. I have no idea how edX will display something like abcstuff{ijk}, but we can certainly allow it in the parsing. Thanks for getting to this - it's been sitting on my todo list for a while...

[ChristopherChudzicki]

Glad it seems reasonable. I think that using abc_stuff_{ijk} is probably not a good idea, but allowing it in the parser seems natural and seems to make the logic simpler: now tensor notation is a strictly "extra stuff" added to end of original edX variable name.

I'm currently happy with the implementation, but I'd like to write some more tests before merging.

[jolyonb]

Indeed. Please include tests like a2bc_d3e__4fg_{ijk}!

[ChristopherChudzicki]

@jolyonb

Indeed. Please include tests like a2bc_d3e_4fg{ijk}!

Maybe you realized that wouldn't work :)

I had trouble implementing normal subscripts and tensor lower_indices simultaneously for the same variable. Since I don't think they would display well anyway, I don't want to worry about it.

I've updated the PR description. The parsing is now, I believe, what you originally envisioned and is fairly clean. So, please review and merge if no issues.