analyse_FadingMeasurement(): results only in NAs if no irradiation steps available

[mcol]

Calling analyse_FadingMeasurement() on an XSYG file from which the irradiation steps have been removed produces this output:

[analyse_FadingMeasurement()]

 n.MC:	100
 tc:	1e-06 s
---------------------------------------------------
T_0.5 interpolated:	NA
T_0.5 predicted:	1e-06
g-value:		NA ± NA (%/decade)
g-value (norm. 2 days):	NA ± NA (%/decade)
---------------------------------------------------
rho':			NaN ± NA
log10(rho'):		NaN ± NA
---------------------------------------------------

We should instead check if all irradiation times are negative (which happens if no irradiation steps can be found), and if so return an helpful message, the current behaviour is very puzzling for any user.