Use PyTorch's dexpm once 1.7 has been released

[brudfors]

Sorry!
Note: there will be a native matrix exponential in PyTorch 1.7, which is in pre-release state I think. Once it's released, we could dispatch expm to either our or their implementation dependeing on the pytorch version. This should give a very decent speedup.

Originally posted by @balbasty in #21 (comment)