Fix an issue with providing astropy Units to calc_datacube

[mperrin]

This fixes an issue I found with the calc_datacube function, which currently errors if provided wavelengths as astropy Quantities:

>>> import numpy as np, webbpsf, astropy.units as u

>>> nrs = webbpsf.NIRSpec()
>>> nrs.calc_datacube(wavelengths=np.array([1,2,3])*u.micron)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)

[... long trace of error text from Astropy trying to write a Quantity into a FITS header]

This fix makes sure the wavelength is converted to a plain float in units of meters before being saved as a header keyword.

No change is needed in calc_psf which already works correctly with units. This fix is just needed for calc_datacube which somehow slipped through the cracks for this.

[obi-wan76]

Looks good, I ran the example in the PR without any error.

[mperrin]

(accidentally pushed a commit to this branch that wasn't intended; I force-pushed to remove it and go back to just the intended commits. I'm going to let the CI run again, and then merge)