Automatically install the OOP C++ header

[Radonirinaunimi]

So far, the OOP C++ header had to be copied manually into where the C header is located or within the project. This allows one to specify a PINEAPPL_C_INSTALL_PREFIX environment variable so that the header is automatically copied at build time. The release workflow has also been adjusted so that this is automatically installed when copying the pre-built binaries:

curl --proto ... | sh -s -- --prefix /my/custom/installation/path

TO DO:

• Update the documentation

[felixhekhorn]

Looks good to me.

• out of laziness I haven't explicitly run the code to check if it actually works - should I do so?
• I would be happy if the OOP interface gets integrated more into the program, but if I remember correctly from Stabilize APIs #328 @cschwan is sceptical about that ... in particular, it was a conscious decision to leave the OOP interface aside; also due to compatibility reasons

[Radonirinaunimi]

* out of laziness I haven't explicitly run the code to check if it actually works - should I do so?

No, not needed. Everything is being checked in the CI. So it was more whether or not you agree with the concept.

* I would be happy if the OOP interface gets integrated more into the program, but if I remember correctly from [Stabilize APIs #328](https://github.com/NNPDF/pineappl/pull/328) @cschwan is sceptical about that ... in particular, it was a conscious decision to leave the OOP interface aside; also due to compatibility reasons

I believe (if I recall correctly) that this discussion was RE the Fortran side.

[cschwan]

Note that the C++ header was installed in the past, but I decided to no longer do that in commit addf958. See the commit for a better way to install it.

However, every interface that we decide to support adds a lot of maintenance burden to us and SemVer restricts us in ways in which we can change it. As far as I can remember we bent the rules quite a bit with C++ and thus I would refrain from calling it an official API, which are C, Python and Rust.

C++ and Fortran are wrappers, both of which use the C interface. We can change the wrappers anytime we want and in any way we see fit, because we assume the people using the wrappers copy them to their source tree. I think this is a good practice and I would keep it that way.