Feature for trivial or custom instrument pack

[drvdputt]

PAHFIT can currently not work without specifying an instrument pack. This is a problem for cases where there is no reasonable instrument.

Example use case: seeing how a PAHFIT decomposition behaves, when fitted to a theoretical dust feature spectrum, for which the resolution of the grid is an arbitrary choice.

Perhaps a sort of 'trivial' instrument pack can be set up, with a fixed resolution and a user-specified range. At a minimum, it should be possible to read an instrument pack from the working directory of the user. They could then make a copy of a trivial instrument pack example, and adjust the wavelength range and resolution as needed