move indices to subscripts

[ChrisRackauckas]

No description provided.

[ChrisRackauckas]

@saschatimme @HarrisonGrodin I think we should instead do indexed variables as one object with indexing. However, I am at least going to here since it gets my newest coolest prototype working.

[codecov]

Codecov Report

Merging #154 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
+ Coverage   95.01%   95.03%   +0.02%     
==========================================
  Files          11       11              
  Lines         401      403       +2     
==========================================
+ Hits          381      383       +2     
  Misses         20       20

Impacted Files	Coverage Δ
src/variables.jl	94.54% <100%> (ø)	⬆️
src/utils.jl	93.65% <100%> (+0.2%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0c833cb...8fce9ca. Read the comment docs.

[chakravala]

The DirectSum package provides extended index-printing for full-alphanumeric indices up to 64 indices, instead of just only having the number indices you get full alphanumeric and it is designed for use with multiple packages including vectors, covectors, partial derivatives, and "epsilons"

https://github.com/chakravala/DirectSum.jl/blob/master/src/indices.jl

Highly recommend using it for printing indices, especially if you need a dual space also and are interested in working with conformal projective geometry (e.g. point at infinity, origin)

It is designed for my own purposes, but I can accommodate your requirements.

[ChrisRackauckas]

I think we can just pull apart the symbol and map the characters directly. The only peculiarity is how to group the subscripts.

[chakravala]

Also, my package provides some special caching for indices so that they can be obtained at precompilation time for maximum performance of printing the indices.

This is helpful when you are dealing with many of permutations of indices for higher order tensors

[ChrisRackauckas]

Index limit fixed by 9ab9df3