Eliminate boilerplate for pointwise functions

[nilsvu]

#4637 adds a lot of lines for adding a single pointwise function along with a compute item and pybindings. I wonder if we can eliminate some of the boilerplate code.. it distracts from spending time on the actual implementation of the function (the added equation is a single line in C++).

With boilerplate code I mean the majority of the lines in #4637 that don't require much (or any) thought, they just follow a fixed pattern:

• Return-by-value and return-by-reference overloads
• Compute tag + simple tag
• Python bindings
• Random-value tests, tag tests, Pybindings test

Here are a few ideas that @nilsdeppe and I discussed:

• Eliminate either the return-by-value or the return-by-reference overload (Can RVO eliminate our return-by-reference overloads? #4648)
• Eliminate the compute tag by computing quantities in a CachedTempBuffer, then expose the individual tags to the DataBox like we do for Variables (Expose CachedTempBuffer tags to DataBox #4649).
• Simplify compute tags (Simplify compute tags #4650)
• Move Python bindings into the file that defines the function so we don't have to add so many files. Have to be careful with enabling/disabling pybindings.
• Test pointwise function pybindings in the C++ test by calling the python function with pypp.