Output option -F mc2 produces a .jnii file

The current master of mcx produces .jnii output files even when specifying .mc2 as output via -F mc2. The other output formats (bnii, nii, hdr, tx3) work though.

We might switch from .mc2 to .jnii in the future. Do you know which output format is the fastest to read?

Steps to reproduce (on Ubuntu 22.04):

`./mcx --bench cube60 -F mc2`

Thank you!
