Support concurrent calls to gridsynth

[jlapeyre]

Currently, concurrent calls to gridsynth cause errors and failures. So it must be called serially.

This requires moving static data into a per-gridsynth call struct (or structs). Two items (need to check if there are more)

• Caches in the diophantine solver
• The big float precision. The precision of all big floating point operations is managed 100% in this application code, as static data. That is, if the big float library stores a working precision, we don't use it.

There are other reasons to pass a struct around calls in the diophantine solver. For example, we likely will want to collect statistics. I already did some of this for the RNG. All of these could be folded into a larger struct.