enable calibration factor for ECAT8

[KrisThielemans]

As we read calibration factors already https://github.com/UCL/STIR/blob/4879cec30a1d364a15462d7b537a749c7923d7d8/src/recon_buildblock/BinNormalisationFromECAT8.cxx#L278-L279 it should be easy to use them (once we know of the cross-calib factor should be used or not).

At present, we set it to 1 https://github.com/UCL/STIR/blob/4879cec30a1d364a15462d7b537a749c7923d7d8/src/recon_buildblock/BinNormalisationFromECAT8.cxx#L194 but this line should be moved to read_norm_data I believe, as otherwise we'd have to duplicate it in the other constructor

[KrisThielemans]

@danieldeidda i think you were going to take this on?

[danieldeidda]

yes I will do that