Use CXOTIME_NOW environment variable as current time if set

[taldcroft]

Description

This updates the CxoTime class to use CXOTIME_NOW environment variable as current time if it is set. This partners with sot/chandra_time#59 to provide broad support for mocking the current time in Python applications. This can be useful for both testing or running a script reproducibly (as if run at a fixed time).

It also improves a test of CxoTime.NOW to provide more margin for running on a Windows VM with long system delays.

Interface impacts

None.

Testing

Unit tests

• Mac

(ska3-flight-2025.0rc2) ➜  cxotime git:(cxotime-now-environment-override) git rev-parse --short HEAD
9864220
(ska3-flight-2025.0rc2) ➜  cxotime git:(cxotime-now-environment-override) pytest                    
========================================== test session starts ===========================================
platform darwin -- Python 3.12.8, pytest-8.3.4, pluggy-1.5.0
rootdir: /Volumes/git
configfile: pytest.ini
plugins: doctestplus-1.3.0, anyio-4.7.0, timeout-2.3.1
collected 254 items                                                                                      

cxotime/tests/test_cxotime.py .................................................................... [ 26%]
.................................................................................................. [ 65%]
.........................................................................                          [ 94%]
cxotime/tests/test_cxotime_linspace.py ...............                                             [100%]

========================================== 254 passed in 1.47s ===========================================

Independent check of unit tests by Jean

• OSX

(ska3) flame:cxotime jean$ git rev-parse HEAD
9864220ab55ec1e27006f31e077a21aee2e01a5c
(ska3) flame:cxotime jean$ pytest
=================================================================== test session starts ====================================================================
platform darwin -- Python 3.12.8, pytest-8.3.4, pluggy-1.5.0
rootdir: /Users/jean/git
configfile: pytest.ini
plugins: anyio-4.7.0, timeout-2.3.1
collected 254 items                                                                                                                                        

cxotime/tests/test_cxotime.py ...................................................................................................................... [ 46%]
.........................................................................................................................                            [ 94%]
cxotime/tests/test_cxotime_linspace.py ...............                                                                                               [100%]

=================================================================== 254 passed in 1.44s

Functional tests

No functional testing.

[jeanconn]

I note that google also told me about freezegun https://pypi.org/project/freezegun/0.3.4/ but I think the env var makes sense for most of our testing applications.