Support single file MRFs, with compact index

[lucianpls]

Copying MRFs is tricky, especially the ones with very large bounding boxes. The canned index format helps, but it has to be uncanned before use. It should be possible to make the canned index an alternative to the array version, which would make the MRF easier to transfer. A side-effect will be that the resulting MRF will be read-only. Another side-effect is that cloning of such a file gets harder. A related feature would be to combine all three components of an MRF into a single one, also enhancing transferabillity.

[lucianpls]

Maybe storing the MRF as is, inside of a tar file is the solution. It doesn't invent a new format, and it could be made to work transparently in GDAL, at least for reading. Writing would be only a bit trickier, it would require first to write the MRF as normal, then do a gdal_translate mrf to mrf with some special options. It could be done in a virtual single pass, using temporary files.

[lucianpls]

Implemented as mrf-in-tar