In the if ~cmplxS... conditional any(k) returns a vector for multi-k
structures, causing a MATLAB:nonLogicalConditional error, fix this
by flattening k first

- Add test with multiple k and n
- Add test with nSpin = nMagAtom
- Add test with nSpin = nMagExt

Test for number of magnetic atoms is already performed at beginning
of function

- Specify n is converted to a unit vector in docstring
- Remove unreachable n/k size error check

- Also move 'extend' -> 'tile' to the top so tile can be used in unused
  input checks
- Also move valid mode check to in unused warning checks so an invalid
  mode isn't searched for in the struct

- Also move errors before warnings

Inspecting varargin is more complex than initially thought,
sometimes it can be a struct (e.g. when called from optmagk)

When called from places like optmagstr, varargin is actually a struct.
Also, the struct contains many other additional parameters to do with
fitting, so before calling genmagstr in optmagstr,
warning('off','sw_readparam:UnreadInput') is called. This stops
sw_readparam from issuing warnings. This also stops the
`spinw:genmagstr:UnreadInput` warning from being triggered, which
is why its name has been changed to UnreadInput, to avoid ugly
unecessary warnings.