Add initialize_panel() so that StreamlitPanelValueAccessor can be cached between runs

[mikeprosserni]

What does this Pull Request accomplish?

This changes the way users will write their streamlit scripts. Instead of writing this:

import nipanel

panel = nipanel.StreamlitPanelValueAccessor(panel_id="sample_panel")

users will write this:

import nipanel

panel = nipanel.initialize_panel()

The initialize_panel() function will create a SPVA, but will cache it so that it isn't recreated every time the script is rerun. Also, the function will automatically get the panelId from the baseUrlPath, so the user doesn't have to provide it.

Why should this Pull Request be merged?

Implements AB#3163243

What testing has been done?

Running sample.py with the new sample_panel.py, I verified in the debugger that the SPVA was not recreated when the rerun button was clicked.

(In order to attach the debugger to the sample_panel.py and _streamlit_panel_initializer.py, I added debugpy as a project dependency. I also added .vscode/launch.json to the .gitignore so that my debug configuration would not be checked in.)

[github-actions]

Test Results

 10 files  ±0   10 suites  ±0   13s ⏱️ -4s
 78 tests ±0   78 ✅ ±0  0 💤 ±0  0 ❌ ±0 
750 runs  ±0  750 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 2d589f1. ± Comparison against base commit 44d18cc.

♻️ This comment has been updated with latest results.