Very basic AdjFloat code generator

[jrmaddison]

Use SymPy for autodiff and code generation. Not intended to be efficient -- the only nod to efficiency is the use of an LRU cache, arbitrarily set to size 256.

np.sin(AdjFloat) etc are annotated.

[dham]

This looks nice but is the test coverage adequate for the huge list of operations that seem to be added above?

[jrmaddison]

This looks nice but is the test coverage adequate for the huge list of operations that seem to be added above?

Tests added.

The new tests hit a divide-by-zero corner case in the power operator. There might be better workarounds than the one I've added (which essentially codes in SymPy), but that would probably be over-complicating things.

[jrmaddison]

A couple of points:

• Code generation uses exec via sp.lambdify.
• The implementation still silently drops annotation of operators which are not handled.
• In principle AdjFloatExprBlock might be used directly for more complicated expressions -- but at that point it would probably be better to switch to a less basic system (e.g. JAX etc).