Adjust manifest and init

Upon changing from setup.py to pyproject.toml, it became apparent
that doing `pip install -e .` in the top directory will miss a
bunch of dataset files because of incorrect pathnames
(there are no such `atomdb/datasets/*/data/*` or
`atomdb/datasets/*/raw_data/*` paths in the repository) in
MANIFEST.in. Also, atomdb.periodic wasn't getting imported in
atomdb.__init__.py. This was causing some file deletions when
switching between branches on my local version of the repository.

Author: maximilianvz

MANIFEST.in

@@ -1,5 +1,4 @@
 include LICENSE
 include README.rst
 
-include atomdb/datasets/*/data/*
-include atomdb/datasets/*/raw_data/*
+include atomdb/datasets/*/db/*

atomdb/__init__.py

@@ -24,6 +24,8 @@
 
 from atomdb.promolecule import *
 
+from atomdb.periodic import *
+
 
 __version__ = version
 r"""AtomDB version string."""