export astropy units and TimeDelta within cxotime

[javierggt]

Description

This is a little thing. Basically, It just allows me to do

from cxotime import CxoTime, TimeDelta, units as u

instead of

from cxotime import CxoTime
from astropy.time import TimeDelta
from astropy import units as u

Testing

• Passes unit tests on MacOS, linux, Windows (at least one required)
• Functional testing

Fixes #

[taldcroft]

This is fine, but since the imports are not used in cxotime.py you can just put them into __init__.py.

[javierggt]

ok, changed it in init.py