U/jct/solving valid lib arg issue

[johannct]

• improve methz treatment
• continue to improve code readability around validLib
• change use of closest_red in fitIR : it should use consiz

[olivierilbert]

It seems that some changes were already merged in 254-zfix=yes-too-slow.
they are fine too me.
In the context in which flux_rescaling is called, better to use the name mass_rescaling.