In-place operators

See https://github.com/autodiff/autodiff/commit/5a8501f36b537a0663105b1288f29801dec495a6

Yet another battle with typing

Fix missing namespace

It works!

* Add Chebyshev2D departure term type

Break up pybind11 interface into separate compilation units (not as beneficial as hoped); still very slow

And JSON I/O

Isochoric derivatives moved into their own namespace

Doesn't work  so far

Not especially fast