A draft CONTRIBUTING document for the EXP source tree [no ci]

[The9Cat]

Ported from the readthedocs version

[The9Cat]

Current status

• Merged in bug fixes from createFromCache, including the new constructors, and moved the Koopman experimental extensions to a new branch KoopmanHankel.
• Checked compilation of devel on Ubuntu 24.04 and on Unity.
• Version is bumped to 7.7.99. We can tag this on merge to main.

[michael-petersen]

At least on my machine (M1 Mac), sqrt is not a constexpr, so I was receiving and error:

EXP/expui/FieldBasis.cc:316:33: note: non-constexpr function 'sqrt' cannot be used in a constant expression
    constexpr double fac0 = 1.0/sqrt(4*M_PI);

This is fixable by allowing fac0 to be computed at runtime; I don't imagine this is a big problem, but perhaps there is a preferred fix? I think we dealt with this before, but I can't remember. I've pushed the simple change.

[The9Cat]

It turns out that this does violate the C++17 standard, even though it's available in GNU. I'd recommend the following workaround:

constexpr double fac0 = 0.25*M_2_SQRTPI;

This is better because it indicates that the quantity fac0 is determined at compile time, not run time.

[The9Cat]

We are probably close to merging this menagerie of fixes to main.

[michael-petersen]

This PR fixes several small bugs, and contains a new-and-improved test scheme. As all tests pass (both on GitHub and in several local cases), I'm happy to approve these changes.