Add cdtools.__version__ and set it up so that there is a single source of truth for the version number

setup.py

@@ -1,11 +1,21 @@
 import setuptools
+import os
+import re
 
 with open("README.md", "r") as fh:
     long_description = fh.read()
 
+# read version from src/cdtools/_version.py
+version_file = os.path.join("src/cdtools", "_version.py")
+with open(version_file) as f:
+    version_match = re.search(r"^__version__ = ['\"]([^'\"]*)['\"]", f.read(), re.M)
+if not version_match:
+    raise RuntimeError("Unable to find version string.")
+version = version_match.group(1)
+
 setuptools.setup(
     name="cdtools-py",
-    version="0.3.0",
+    version=version,
     python_requires='>3.8', # recommended minimum version for pytorch 2.3.0
     author="Abe Levitan",
     author_email="abraham.levitan@psi.ch",

src/cdtools/__init__.py

@@ -6,6 +6,8 @@
 
 __all__ = ['tools', 'datasets', 'models', 'reconstructors']
 
+from ._version import __version__
+
 from cdtools import tools
 from cdtools import datasets
 from cdtools import models

src/cdtools/_version.py

@@ -0,0 +1 @@
+__version__ = "0.3.1.dev"