Split renfxx.h

[saraedum]

There were a number of minor issues with renfxx.h:

• IDEs and tools such as standardese get confused by the .h ending and assume that this is a C header. There's always ways to tell these tools but it's easier to just change the suffix.
• It is not good for build times to always include all of e-antic. We already had renfxx_fwd.h (now renamed to renfxx_fwd.hpp) for this but now one can also choose to only include the parent or the element of e-antic as needed.

I sneaked in an ODR violation fix in cppyy.h (now renamed to cppyy.hpp.) A non-template definition in a header file should be marked inline so it can be deduplicated.

[saraedum]

@videlec I had originally meant to put this only on the documentation branch but since I need additional operator= to improve normaliz performance, I don't want to create too many conflicts here. I'll merge this into the documentation branch later.

[codecov[bot]]

Codecov Report

Merging #174 (c2ed29a) into master (f133fee) will increase coverage by 0.09%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #174      +/-   ##
==========================================
+ Coverage   90.11%   90.21%   +0.09%     
==========================================
  Files         104      105       +1     
  Lines        1953     1952       -1     
==========================================
+ Hits         1760     1761       +1     
+ Misses        193      191       -2     

Impacted Files	Coverage Δ
libeantic/e-antic/cereal.hpp	100.00% <ø> (ø)
libeantic/srcxx/renf_elem_class.cpp	90.49% <ø> (ø)
libeantic/e-antic/renf_class.hpp	100.00% <100.00%> (ø)
libeantic/e-antic/renf_elem_class.hpp	100.00% <100.00%> (ø)

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