Convert mhn_tools.tbx to mhn_tools.pyt

[tk-oleary]

the current mhn_tools.tbx file is not reviewable in git (must open the app to see), and setting up tool validation within ArcGIS Pro is inefficient. Would be an improvement to convert mhn_tools into a .pyt file so that its contents are searchable and reviewable in git, and tool validation code can be shared between tools.

https://pro.arcgis.com/en/pro-app/latest/arcpy/geoprocessing_and_python/creating-a-new-python-toolbox.htm