Revise installation instructions and C++ linkage

[CMacM]

As pointed out by @c-d-leonard, when the module is installed via pip Galsim C++ libraries must be manually built and linked.

Galsim documentation suggests that this is only required via pip installation and with conda installation it is handled automatically.

Suggested changes:

• Add environment.yml to build conda env from which will install and automatically configure Galsim C++ libraries.
• Expand @c-d-leonard 's description to reflect why manual linkage is required.
• Investigate setup.py to see why linkage is failing (I believe I tried to add code to run this linkage step automatically when BATSim is built).

[CMacM]

It appears the old installation via pyproject.toml and setup.py was also the cause of a bug in BATSim casuing Galsim C++ surface brightness profile subclasses not to be recognised in the BATSim C++ layer.

The reason this was happening on some systems and not others was because it was affected by how Galsim was installed, when Galsim was installed, and what compilers/libraries were already built/linked on the system.

Because Galsim libraries must be built and linked manually when installed via pip, I am moving the recommended install route to conda.

I will add alternative instructions for a pure pip installation at a later date, but this will require a fair amount of work so for now, I suggest you use conda/mamba to build and install BATSim. If you wish to use pip, you do this at your own peril 😆