Add __version__ support to nipanel.

[mjohanse-emr]

What does this Pull Request accomplish?

Similar to a recent change in nitypes, this adds the ability to query the package version using nipanel.__version__.

Why should this Pull Request be merged?

So that clients of this package can query the version.
Consistency with nitypes

What testing has been done?

I ran the following code in the poetry venv with these changes and it returned the correct version:

>>> import nipanel
>>> nipanel.__version__
'0.1.0'

[github-actions]

Test Results

 10 files  ±0   10 suites  ±0   16s ⏱️ -6s
 96 tests ±0   96 ✅ ±0  0 💤 ±0  0 ❌ ±0 
930 runs  ±0  930 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 989f0a1. ± Comparison against base commit e2793bc.

♻️ This comment has been updated with latest results.