Make luminosity unit configurable

[clelange]

Fixes #26

Rather than using

if type(lumi)==float: 
        cms_lumi = f"{lumi:.0f}" if round_lumi else f"{lumi}"
        cms_lumi += " fb^{#minus1}"
elif type(lumi)==str:
        cms_lumi=lumi

as suggested in #26, I think that only changing the unit itself but without the -1 exponent might be safer to prevent people from accidentally using different ^-1. Let me know what you think, @pfs.

[pfs]

thanks a lot @clelange that's more elegant and will cover the needs