C++17: simplify dynamic MPI wrappers

[olupton]

In #834 we will require C++17. This will allow TODOs like

CoreNeuron/coreneuron/mpi/nrnmpi.h

Lines 71 to 73 in 0574812

	// This could be done with a simpler
	// template <auto fptr> struct function : function_base { ... };
	// pattern in C++17...

to be addressed, simplifying the code.

There is an internal reference to consult too.