Consolidate MetafileImporter and SequenceIO

[lassoan]

MetafileImporter module was moved here from Sequences module (which is now part of Slicer core in Slicer-4.11). Now we have two modules for reading PLUS sequence files (image+transforms) from mhd and nrrd files.

SequenceIO is newer, it uses code from IGSIO instead of its own logic, and handles color images and compressed video. MetafileImporter might be faster in parsing large text files (tens of minutes of recording of 6-8 transforms).

Parsing speed of MetafileImporter and SequenceIO has to be compared on large files. If there is no significant difference then MetafileImporter module can be simply deleted. If MetafileImporter is much faster then its parsing code should be ported to IGSIO before the module is removed.

There is also a difference in what file formats are supported. MetafileImporter accepts seq.mha and simple .mha, but since these may interfere with plain volume sequence and volume reading, it is probably better not to offer these file extensions (keep current SequenceIO implementation).

This is not urgent, it is enough to work on this if a related issue comes up.