protect more undefined symbols, start work on cFetch
Remove FUNCTIONPARSER_INSTANTIATE_TYPES from fpoptimizer.cc.

Instead of explicitly instantiating _all_ of FunctionParserBase,
just explicitly instantiate the Optimize() function for each
of the numerical types which are supported.

This fixed a duplicate symbol linking error for me on a GCC/OSX
system, but it will need to be revisited if fpoptimizer.cc ever
gets rebuilt...