Adds Basic Math

[ryanmrichard]

Is this pull request associated with an issue(s)?
No.

Description
This PR adds the Einstein-summation-convention DSL to TensorWrapper and implements it for basic
math operations.

TODOs
Finish adding DSL terms to Parser. Implementing the Eigen executor. Tests. Documentation.

[ryanmrichard]

I did not scope this PR well. I forgot how many pieces need to be in place before something like C("i,j") = A("i,j") + B("j,i"); just works. At present, C("i,j") = A("i,j") + B("i,j"); works (no permutation) and I'm going to call that a good stopping spot for this PR. The next PR will make permutations work (I left some infrastructure in this PR associated with those efforts as I intend to build off this PR). Highlights of this PR:

• Adds rebind to the allocator (will need reallocate when we start to support different backends)
• Adds addition and addition assignment to buffer (for now assumes layouts are the same before and after).
• Stubs for permute and permute assignment (don't work because layouts won't be the same)
• Adds ability to label buffers, so they can be composed like tensors.
• Can print buffers.
• Adds DummyIndices class for string parsing the labels on tensors, layouts, etc.
• Adds Labeled class for extending the DSL to included objects paired with dummy indices.
• Adds PairwiseParser to walk the abstract syntax tree and evaluate it in a pairwise manner.

Once this passes CI it's r2g (gcc has warning clang didn't).