Fixes for Neurodamus and Mac instructions

[ferdonline]

Neurodamus, when building in mac suffered from two issues:

• always assume a brew Python
• link to neurodamus-core with "run" dependency, which made
  it load modules and have variables overwritten. See
  [BSD-118][macOS][python][dyld] spack install fails because of static linking #757 (comment)

This PR

• the hardcoded Python version was removed and instructions how to use BlueBrain spack with Mac were added.
• the dependency was changed to "link", (doesn't help) - in the future we should remove the dependency altogether and use submodule

[matz-e]

Thanks for getting this started! I'm thinking we could probably move this section up underneath the Ubuntu ones?

Suggested change

	## Building software in MAC
	<details>
	<summary>
	Install software in MAC. Use BREW as a source of binary packages
	</summary>
	In MAC the build process is very similar to Ubuntu. To avoid building
	the whole stack from source we can likewise use a package manager.
	For that end we have successfully used Brew. We also provide a skeleton
	packages.yaml that you should review and adapt to your needs.
	Before starting, please install brew and the required packages.
	If you require Python please install a version dowloaded from
	Python.org, as several issues have been found with "brewed" Python
	## Building software on OS X
	<details>
	<summary>
	Install software on OS X, using Homebrew for binary packages
	</summary>
	On OS X the build process is very similar to Ubuntu. To avoid building
	the whole stack from source we can likewise use another package manager to provide precompiled binaries.
	To that end we have successfully used Homebrew. We also provide a skeleton
	`packages.yaml` that you should review and adapt to your needs.
	Before starting, please install brew and the required packages.
	If you require Python please install a version dowloaded from
	Python.org, as several issues have been found with Homebrew's Python

Which packages are required? Also, rather than downloading an external Python, Spack could just build it…

[alexsavulescu]

Which packages are required? Also, rather than downloading an external Python, Spack could just build it…

Not a bad idea, if we nail all its dependencies beforehand, takes ~6 mins to build. @ferdonline what do you say about adopting this strategy? Brew's Python is also causing issues with wheels as far as you were pointing out.

[ferdonline]

The issue is not the build time, even though is not negligible. The issue is that it will probably end up with the same issues as brew Python... Python from Python.org are the standard ones, build with well tested settings and env vars for retro versions compatibility. I think it should be the default. Mac users only have to download and install, not like the linux world.

[alexsavulescu]

This can come in handy in a future update of our spack :

• https://spack.readthedocs.io/en/latest/build_settings.html

[alexsavulescu]

spack#15158

[matz-e]

Not in our tree yet :(