Spectrum.smooth does not preserve velocity frame and doppler convention.

[mpound]

Describe the bug
When you smooth a Spectrum, the resulting Spectrum does not inherit the velocity_frame, doppler_convention, observer, or _observer_location of the original spectrum.

How to Reproduce

1. velocity_frame, doppler_convention, and observer

from dysh.fits import GBTFITSLoad
from dysh.util import get_project_testdata
f = get_project_testdata()/'TGBT21A_501_11/TGBT21A_501_11_scan_152_ifnum_0_plnum_0.fits'
g = GBTFITSLoad(f)
s = g.getspec(1)
print(f"{s.velocity_frame=}\n{s.doppler_convention=}\n{s.observer=}\n{s._observer_location=}")

s.velocity_frame='itrs'
s.doppler_convention='optical'
s.observer=<ITRS Coordinate (obstime=2021-02-10T07:38:37.500, location=(0., 0., 0.) km): (x, y, z) in m
(882593.9465029, -4924896.36541728, 3943748.74743984)
(v_x, v_y, v_z) in km / s
(0., 0., 0.)>
s._observer_location=None

t = s.smooth("gaussian",16)
print(f"{t.velocity_frame=}\n{t.doppler_convention=}\n{t.observer=}\n{t._observer_location=}")

t.velocity_frame='fk5'
t.doppler_convention='optical'
t.observer=None
t._observer_location=None

2. observer_location

from dysh.spectra import Spectrum
from dysh.coordinates import Observatory
# use Spectrum `s` from example above.
s2 =  Spectrum(s.flux,observer_location=Observatory["ALMA"],meta=s.meta)
print(f"{s2._observer_location=}")

<EarthLocation (882593.9465029, -4924896.36541728, 3943748.74743984) m>

t2 = s2.smooth('gaussian',16)
print(f"{t2._observer_location=}") 

t2._observer_location=None

Environment

• Dysh version 0.4.x
• Python version 3.10
• OS Ubuntu 22.04

[astrofle]

@mpound I'm a bit conflicted about this. The two constructors for Spectrum can result in different behaviors which are not consistent. Spectrum.make_spectrum() will not add a observer_location attribute as is. Should we add that option or remove observer_location from Spectrum.__init__?

[astrofle]

Some notes:

• Removing Spectrum._observer_location does not break any tests.
• All Spectrum objects are created using the Spectrum.make_spectrum class method.

Based on this, it seems safe to remove Spectrum._observer_location and keep Spectrum._observer.

[astrofle]

This was solved by PR #436