Speed up build_ellipse_model

[taranu]

Like #1288 (which seems abandoned), this speeds up build_ellipse_model to address #1168 . However, it is entirely written in Cython and does not require a separate shared library. In local testing on Python 3.12, it's about 130x faster than the current pure Python implementation. I also made the tolerance for one of the unit tests much stricter, because I found that it was passing even when the output was incorrect.

There is a potentially premature optimization to specialize the function depending on whether harmonic arrays are passed or not. The C code generated on my system looked correct, but I didn't actually find any difference in performance between high_harmonics=False and True.

I also attempted to parallelize it with Cython's prange. However, in my testing on multiple Linux systems, I found it never actually used more than one thread. Additionally, I couldn't figure out how to allow for a num_threads=None value in the Cython build_ellipse_model_c function, since it's typed as an int. Thus, I'm making this a draft PR to start. If someone more versed in Cython/prange could have a look at that, it would be helpful. Otherwise, the parallel support could simply be dropped.

[taranu]

I tested this on two different systems and found that num_threads does nothing because it was compiling without -fopenmp and not linking against libomp (or libgomp). After manually recompiling, parallelism works fine - or at least, runtimes are about ~3.2x faster with 4 threads.

I see that there are openmp helpers in extension-helpers, which is a requirement and enabled in pyproject.toml here, but I'm unclear on how to encourage pip install to add the relevant compiler flags. @astrofrog, could you advise?