Sadly, C doesn't have constructors so I usually forget to initialize
structures, which lead to segfaults since they are accidentally freed later.

This way I need to modify one function instead of hunting around for various
pieces in the parameters.

There are a couple of issues with the Zeldovich approximation:
- computing the integrals on-the-fly is ridiculously slow, even for density-density
- on the other hand, GSL doesn't have support for 3D interpolations
- the midpoint approximation has about the same accuracy (see arXiv:2011.06185),
  and only requires 2D interpolation
- lastly, the code wasn't using it anymore

Hence, I just removed the option altogether from everywhere.

Leaving in the paralellization causes the code to hand when running the OpenMP
loops, so that one has to be disabled then.  Running some tests, it seems the
parallel multidimensional sampling is actually slower than just using OpenMP,
so it'd be ideal to disable Cuba paralellization, which is what this commit
achieves by just setting the `CUBACORES` environmental variable to 0, which
works.
Strangely enough, if I directly call `cubacores` from the C code, and set
`CUBACORES` to 0, which should have the same effect, it crashes the Cython code
with a very strange error:

python3.7: ./src/common/Fork.c:100: cubafork: Assertion `socketpair(AF_LOCAL,
SOCK_STREAM, 0, fd) != -1 && (pid = fork()) != -1' failed.

I've tried experimenting with the Cython `nogil` option, but to no avail, so
this is more of a hack than anything (that works!).

- hubble_rate
- hubble_rate_conformal
- hubble_rate_conformal_derivative

Apparently the code compiles just fine without `-lgomp`.
Furthermore, the wrapper has no use for the `libconfig` library for now, so the
wrapper can safely be built without it by defining a macro `COFFE_CYTHON`
(maybe an alternative name would be better).

Now it's quite simple: it accepts the desired order of interpolation (1 means
constant + linear term), and outputs the coefficients as well as the chi
squared statistic.

Useful for the following members:
- interp_method
- covariance_interp_method
- pk_type

This way we can pass a bias with an arbitrary number of parameters to the
setters.

This will come in handy later when implementing the bias in each redshift bin

This is to help debugging, since some of the structures probably aren't yet
free'd properly.

Also updated the compiler flags so they detect Cuba properly.