What to do about spaces in header entries with spaces in keys?

[jdtournier]

This affects images produced by fixelcfestats, as discussed here.

I think we'll need to improve the MatLab handling (clearly there's no issues with handling these within the C++ code), but I also think it might be better to avoid spaces in key names in general, and modify fixelcfestats to avoid spaces in the first place. This should be OK to do, as far as I can tell, these headers entries are not queried anywhere else in the code.

Thoughts?

[Lestropie]

I'd do both: Fix the MatLab handling on master, and remove the the spaces from header keys at write time on dev.