Inclusion of the extreme-UV part both for stars and dust

[galaxyumi]

The functionality for dealing with the EUV parts of the SEDs in the BEAST has not been maintained well. We need to update/add back those functionalities.

@karllark will include dust physic models (starting with Draine's models), which goes down to the X-ray part, in the dust_extinction package. Then we will make the merging between a dust physic model (for wavelengths shorter than the FUV ) and an observed extinction curve (for wavelengths equal or longer than the FUV) happen on the fly in the BEAST.

Some of the EUV filters can be added into the BEAST filter table for the user's convenience.

The wavelength range of a spectrum in the current SpecGrid generation starts from 1000 A. We want to fix this, so model spectra have a full wavelength range even in the EUV regime.

[karllark]

The dust_extinction package has been updated to include various dust grain models including the Weingartner & Draine (2001) models. See https://dust-extinction.readthedocs.io/en/latest/dust_extinction/model_flavors.html#grain-models