expand parameter_t (no typedef as alias for lengthy class names, except perhaps ocasionally at file scope)

Merge branch 'precompute' into develop - before starting to really work on the precomputation mechanism

Removed inclusion of FitKernel.h from FitSuite.
This required one nontrivial change: The pointer-to-implementation
must be a simple pointer, not a unique_ptr. If pimpl is a
unique_ptr, then no instance of FitSuite can be created without
knowing the size of FitKernel.