Adds vcs versioning for git_nxdl_version

dev_tools/globals/nxdl.py

@@ -8,14 +8,42 @@ class definitions (the NXDL namespace).
 relative URI "../nxdl.xsd" which means validation
 can only be done for NXDL files in subdirectories.
 """
+import os
+from subprocess import CalledProcessError
+from subprocess import run
+from typing import Optional
 
 from .directories import get_nxdl_version_file
 
 XSD_NAMESPACE = "http://www.w3.org/2001/XMLSchema"
 NXDL_NAMESPACE = "http://definition.nexusformat.org/nxdl/3.1"
 
 
+def get_vcs_version(tag_match="*[0-9]*") -> Optional[str]:
+    """
+    The version of the Nexus standard and the NeXus Definition language
+    based on git tags and commits
+    """
+    try:
+        return (
+            run(
+                ["git", "describe", "--tags", "--long", "--match", tag_match],
+                cwd=os.path.join(os.path.dirname(__file__)),
+                check=True,
+                capture_output=True,
+            )
+            .stdout.decode("utf-8")
+            .strip()
+        )
+    except CalledProcessError:
+        return None
+
+
 def get_nxdl_version() -> str:
     """The version of the NeXus standard and the NeXus Definition language"""
-    with open(get_nxdl_version_file(), "r") as fh:
+    version = get_vcs_version()
+    if version is not None:
+        return version
+
+    with open(get_nxdl_version_file(), "r", encoding="utf-8") as fh:
         return fh.read().strip()