Spectral fitting with the BKG detector in OSPEX during times with the attenuator inserted

[afbattaglia]

With the pull-request that has been accepted on 5th April 2022, it is now possible to perform spectral fitting with OSPEX just by using the BKG detector. This is particularly useful in order to get, for example, temperature and emission measure during times in which the attenuator is inserted. The keyword no_attenuation has to be used for this purpose. We tested this feature with the SOL2021-10-28 X class flare and it seems to work properly.

Anyway, the current version is temporarily. Indeed, there are some stuff that we still need to take into account. For example, we need to adapt the normalization of the observed spectrum that, with the current version, can be slightly off. Is there something else we should also consider, @grazwegian ?

[samaloney]

@afbattaglia / @grazwegian still an issue?