MCPL release 2.2.6

In addition to various minor maintenance updates, this release brings a small fix from @g5t to the implementation of the mcpl_merge_outfiles_mpi function, which lowers the peak disk usage at the end of an MPI job (cf. #119 and #120).

This function is currently known to be used in McStas and McXtrace, so users of those applications will immediately benefit from this improvement.

Full Changelog: v2.2.4...v2.2.6