Feature added - spec.dopcor()

hoki v1.1.0-alpha

A small and basic doppler correction feature was added - dopcor.

The spec.py module was created - this is where dopcor resides. We enticipate that spec will contain an extensive suites of spectroscopic utilities in the future to use with the BPASS synthetic spectra (and potential other spectra in similar formats).

Backwards compatibility should be conserved - please raise an issue if that is not the case.