Refactor doctests, enable documentation of dunder methods by default in API docs (#368)

* refactor docstrings for `StringMatrix`

* enable dunder methods for completeness

* add docstring for `__version__`

Author: alexlancaster

src/PyPop/__init__.py

@@ -142,6 +142,7 @@
 
 try:
     __version__ = metadata_lib.version(__pkgname__)  # use the installed version first
+    """PyPop version. If installed, this is the package version, otherwise it returns repository version."""
 except metadata_lib.PackageNotFoundError:
     from setuptools_scm import get_version