install Python only if not yet installed

[scriptPilot]

Closes #234

Root cause: Python might be installed already by Xcode developer tools or other sources before

[scriptPilot]

It figures out it is even a bit easier.

Homebrew installes Xcode developer tools first which installs Python which installs Python TK.

So it must not be installed again in the install script :-)

[w8sl]

"brew install gdal" will install Python 3.12 anyway, as dependency. scikit-fmm for Python 3.12 has not been relased yet. Therefore, it is necessary to force the whole install into python3.11 (or 3.10 if preffered). See comment:
#210 (comment)

[scriptPilot]

Ok, well I will try the linked steps by step - thanks for that.

I think it would be really helpful to have the same Docker image for all platforms in the future.

Only Docker will be required - and maybe XQwartz for MacOS (tutorial).

[w8sl]

I am trying to update install instructions for 1.40 on my fork

[scriptPilot]

My attempt to install instructions. I don't know how to install scikit-fmm on Arch. Install_Instructions.txt install_mac.sh.txt requirements.txt

Would be helpful to have the installation instructions per OS and later for Docker in the README file.
I spent a couple of hours to try on MacOS as the good instruction wasn't found before 😬