Auto-generate Fortran headers

[RobertPincus]

The *-kernels/api directories contain Fortran modules that describe the API for each of the C-compatible kernels. These are used in the auto-generation of C++ headers (see #352, #354) and when calling kernels implemented in other languages (e.g. CUDA) from Fortran.

These kernels are currently maintained by hand. It would be better to auto-generate them from the Fortran source.