Remove autodiff overhead

[calcmogul]

The autodiff isn't fast enough for embedded environments.

Option 1: Code generation

Generating a linear sequence of stack operations would be faster. It should walk the autodiff tree as usual and write out the C++ expressions instead of computing them. A Doxygen comment should be generated for each function.

Option 2: Expression templates

Since the overhead comes from dynamic dispatch, we could use expression templates instead, though that could get complicated and increase compilation times.

https://en.wikipedia.org/wiki/Expression_templates