various quality of life attempts for Muscle

[starsfordummies]

Based on the remove permutedims() PR (#58 ) so finalize that one first

Changes are all in QOL.jl for simplicity, can cherry pick.

• Shorthand * for binary_einsum

• Add helper function contract(::Tensor, ::Tensor, inds) to contract tensors over specified indices

• More human printing for tensors, instead of printing all values focus on their indices, in my experience works best for debugging algorithms

• Various helper functions to make all the rest work

[jofrevalles]

Great! Have you seen speed-ups with these changes?

[starsfordummies]

the only performance gain is with removing the permutedims of #58 - the rest is just helper functions, they don't need to be public or anything but I find that they make programming much less tedious

the idea is that if we have two tensors a[i,j,k] and b[j,l,m] we can compute (a*b)[i,k,l,m] by writing a*b and that's it (similar to what other libraries do..) .

With contract(), if we have two tensors like a[i,j,k] and b[l,m,n] and we want to contract the legs k and m, we do contract(a,b, :k, :m) and it does all the relabelling and contracting for us, returning ab[i,j,l,m]