ENH: refactor mio module imports for greater beauty

Remove unused; use bracket instead of backslash continuation.
API: remove use of find_mat_file and deprecate

Deprecate for removal of utility function in next version
API: raise error for >2D array and Mat4 format

Threatened deprecation now an error.
API: make oned_as='row' default for Mat5

Previously `oned_as='column'` was the default; deprecation threatened
for a long time now.