Check directory looks like a copy of CSET before installing it

[jfrost-mo]

What problem does your feature request solve?

Currently if you leave the path box blank when you install local CSET it will try and copy your entire filesystem. This is undesirable.

Describe the solution you'd like

We should check if the path given resolves to a directory that at least minimally looks like a CSET project, maybe by checking it contains a pyproject.toml file. If it doesn't we give a clear error message and fail the task.

Describe alternatives you've considered

It is a little annoying having to specify this path, as it will be per-user, and thus can't be preconfigured. We could move the workflow out of the cset-workflow folder and into the top level of the repository. That way the CSET source would be copied on cylc install, and we could build from the known workflow install directory.