MRT: Use bottleneck for nanmedian when available

[pllim]

Description

This pull request is to try bottleneck for #233

RT: https://github.com/spacetelescope/RegressionTests/actions/runs/28265017532 , https://github.com/spacetelescope/RegressionTests/actions/runs/28610438618

[pllim]

Timing below is kinda iffy because they ran on VMs, so this only gives a rough estimate. The Python 3.14 job does not pull in bottleneck.

Without this patch (master branch)

Python 3.10: 14.05s call acstools/tests/test_wfc_findsat_mrt.py::test_WfcWrapper

With this patch

Python 3.10: 9.87s call acstools/tests/test_wfc_findsat_mrt.py::test_WfcWrapper

[pllim]

After switching away from Git LFS (#259), the runtime without bottleneck was 9s. With bottleneck, it went down to 6s.

[pllim]

After some code clean-up (#262), here are the updated timings. Note that these were all run on VMs, so take it with a grain of salt.

• Without bottleneck (master): 13.39s
• With bottleneck (this PR): 10.84s

[pllim]

So far, this is the only one that gives visible performance boost and actually don't change results. So, I am taking it out of draft for formal review.