Installation instruction updates

[thjsal]

Things to be fixed/updated in the installation instructions:

• MultiNest should be installed before installing mpi4py using conda install mpi4py, or the path to mpif90 compiler should be specified to be not the apparently brokenmpif90 created by mpi4py installation.
• GSL can be also installed with conda install gsl, and in some cases (especially for newest Mac) it seems even required to get gsl linked correctly with X-PSI (unless modifying the from source instructions). Conda installing gsl seems to work in Linux-machines too. So we can probably advice installation of GSL to be always using conda.
• The version of Sphinx needs to be specified to be below 7.0.0, since otherwise compiling the documentation causes a "'style' is undefined" error as in https://github.com/kivy/kivy/issues/8230. Alternatively, the notebooks (apparently Emitting_patterns_2Dprojection.ipynb at least) should be updated to be compatible with the latest sphinx version.

[evertrol]

You may also want to add another little part to the clean-up command at the bottom of the XPSI installation section (just above the "Documentation" section), to remove the generated rayXpanda object files:

rm -r build dist *egg* xpsi/*/*.c xpsi/include/rayXpanda/*.o

(it may complain the files are not found if the included rayXpanda package was not used.)

Note that this concerns the object files, not the C source files, since the C files are the actual source files here (i.e., there are no Cython .pyx files). And since these are compiled in-place (see the sub.call() lines in setup.py), the object files are left in the source directory. Which could cause problems during a new build.

[thjsal]

We should also add sudo apt install mpich to instruction, to make sure mpif90 is installed.