Add plasma module back into __init__.py

This avoids a regression effect against the documentation in README.md.

Author: DanShort12

parametric_plasma_source/__init__.py

@@ -1,5 +1,7 @@
 """
 __init__ for parametric_plasma_source.
 
-Does not collect any imports as package may not be built yet.
+Does not collect imports from plasma_source as package may not be built yet.
 """
+
+from .plasma import Plasma