BF: allow any sequence for variable_names and mat4

Mat4 `variable_names` parameter that was not a list raised an error;
cast iterable to list to allow tuples etc as input.
TST: io/matlab: test strings as variable_names parameter
Merge pull request #2498 from matthew-brett/allow-tuple-varnames-mat4

BUG: io.matlab: allow any sequence for variable_names and mat4