Unirradiated background from film used for calibration

[LuisOlivaresJ]

About the to_dose method, users should be allowed to use the unirradiated background from a film used for calibration, instead to force them to scan an irradiated and an unirradiated film to account for self-development. The user should be warned that this new approach increase uncertainty when the lapsed time from calibration to a new irradiation is more than a month. Additionally, another tif file could be used with an unexposed film or an irradiated film with a known dose to scale the calculated dose.