Add shell tool

[hiker]

Adds a shell tool as a convenience for other tools (e.g. lfric build scripts run nf-config, which can now use this).

#28 should be merged first, this branch is based on #28.

[hiker]

I had to change the 'check if available' parameter (which was a single string, e.g. --version to take a list - across the various shells there was no way of testing them (e.g. dash and sh do not have any version output parameter). So in the end I run a simple echo command, and this required a change of the parameter from a simple string to a list of strings.

Additionally, I noticed that tool_repository.get_default_tool could return a tool that is not actually available on a machine, which with the new shell command becomes more important. So I've also updated this to return the first tool that is available (so e.g. on a machine without bash .... if that should exist ... it will now try to use sh, ksh, dash instead.