_do_data_assimilation passing basename, which has the extension

In _do_data_assimilation `os.path.basename(da_script)` is passed as the subname to run_or_cmd

https://github.com/ESMCI/cime/blob/87d67c201247b1b91677f114f32fa8c0a6f52b18/CIME/case/case_run.py#L422-L436

If your da_script has an extension, e.g assimilate.py the run_or_cmd function ends up looking for `def assimlate.py` in the file rather than `def assimilate`

I think `os.path.splitext(os.path.basename(filepath))[0]` lets you have a script.py (or script with no extension) and still find the `def script` in the file.

```
>>> filepath='/users/hkershaw/assimilate.py'
>>> os.path.basename(filepath)
'assimilate.py'
>>> os.path.splitext(os.path.basename(filepath))[0]
'assimilate'
```

I'm not sure if there is a CIME convention to not have extensions (e.g. .py) on scripts that could be imported.