Add script for creating environment module files

[dschwoerer]

This program, bin/add-mod-path.py provides
a convenient way to create modules for the environment module for
BOUT++ that sets PYTHONPATH, PATH and BOUT_TOP, so that switching
between different BOUT++ installations is easy.
bin/add-mod-path.py stores the loaded modules as prerequesit, so that you
don't accidentially mix e.g. mpi versions. Feel free to edit the
crated module file, if there are non-needed dependencies.

bin/add-mod-path.py can be used interactively, or the location where to
create the modfiles, the name and the BOUT++ directory can be
passed:
bin/add-mod-path.py [modulepath [name [bout-top]]]

If modulepath is not passed, the script checks whether any of the
paths in $MODULEPATH are writeable, and asks which of them to
choose. If non are writeabel, and you can or want not to run as
root, you can extend this. Create a new folder and at it to your
.bashrc e.g. with:

 mkdir ~/.module
 echo 'export MODULEPATH=$HOME/.module:$MODULEPATH'>> ~/.bashrc
 source ~/.bashrc

name can by any name by which you will be later able to recognice
this specific bout installation, e.g. 3d-metrics. You can later
load the module with module load bout/name e.g. module load bout/3d-metrics.

bout-top is the root directory of the BOUT++ installation. Note that
it only works with in-source-builds.

Might help with #2010

[ZedThree]

Neat! This is pretty cool. Just a few comments. The module name should probably bout++ rather than bout. Please also run your favourite PEP8 tool over this.

[ZedThree]

Might be clearer to use either os.path.basename or pathlib.Path.parent?

[dschwoerer]

I tried basename first, but that doesn't work, at least the one from os.path: https://docs.python.org/3.5/library/os.path.html#os.path.basename
If there is a different one, with different behaviour, I wouldn't call that intuitive.

I only found parent in purepath, but that does anyway the same I do, but by having an additional layer, I would assume it is better then that, so I think it is easier to just do it myself, but if you have prefer the parent one, I don't care that match ...
https://docs.python.org/3.5/library/pathlib.html?highlight=pathlib#pathlib.PurePath.parent

[ZedThree]

Well it's more that using a named function makes the code more explicit and easier to understand, even if it's doing exactly the same thing under the hood. This isn't something where we need to squeeze out every last drop of performance, so prefer to make code readable.

(btw pathlib.Path inherits from PurePath, but can do actual IO operations, PurePath doesn't need an actual filesystem)

[ZedThree]

Why not just use raise RuntimeError?

[ZedThree]

Typos:

Suggested change

	help='''can by any name by which you will be later able to recognice
	help='''Can be any name by which you will be later able to recognise