use c++20 concepts to specify requirements for template functions and classes

[cwsmith]

See #42

The user facing interface functions:
https://github.com/SCOREC/meshFields/blob/a87c33c1d4f9806d45c91d2521fe7e5dad67a388/src/MeshField.hpp
are the most important ones to harden with the addition of concepts. After that, we need to go down the rest of the stack.

This should make all the interfaces easier to use and result in fewer bugs.