Implementation for Tensor Covariant Derivatives and Substitutions

[Jason Ross]

Hello,

I asked a question previously about doing tensor calculus in Sympy. I ended up implementing basic tensor substitution and covariant differentiation in my branch.

Here's a gist showing how these features can be used to solve some basic tensor calculus problems:

https://gist.github.com/Jason-S-Ross/f33ea4aa80cc2577be0bff3dbe19c765

I don't personally have the mathematical background to go much further than this, but I wanted to share in case someone who does takes interest.

Cheers,

Jason