Deprecate 'kind' and 'path' in favor of 'fname' in the layout reader

[mscheltienne]

Closes #11483

As discussed in #11483, those 2 arguments don't play well and could be replaced by a common explicit fname argument.

[larsoner]

1. All existing user code currently will have either read_layout(name, ...) or read_layout(kind=name, ...). It would be nice if at least the first one (which might even be the most common!) continued to work without a deprecation warning if possible. So what if we deprecate differently by doing:

   Suggested change

   	def read_layout(kind=None, path=None, fname=None, *, scale=True):
   	def read_layout(fname=None, path=None, scale=True, *, kind=None):
   	if kind is not None:
   	warn(...)
   	...
   	if path is not None:
   	warn(...)
   	...

   That way read_layout('Vectorview-all') works on 1.3 and also in this PR without any warning.

2. Since the current default for path is None, we need to use something other than None as the default now to know if a user has actually passed the param or not. otherwise code like this:

   read_layout('VectorView-all', path=None)
   

   works on 1.3, does not warn the user for passing path=None in 1.4, yet will raise an error about the kwarg not existing come 1.5. We could for example use path='' (I think it's safe to assume no user has done path='' in their code) and do:

   _validate_type(path, ('path-like', None), 'path')
   if path != '':
       warn(...)
   

   as our user-passed-so-need-to-warn check.

[mscheltienne]

Good points, it should be better now, thanks!

[mscheltienne]

I don't think the CI failure is related to the PR.

[larsoner]

LGTM, will merge once CIs come back happy again (other than the notebook one, which I'll investigate separately)!