fix: lazy imports for matplotlib & gdstk (#161)

only require matplotlib & gdstk for functionality where we actually use them

contributed by @gerardvt

doc_utils.py

@@ -5,7 +5,6 @@ import subprocess
 from typing import Optional
 
 import chevron
-import matplotlib as mpl
 
 from config import Config, DatasheetConfig
 
@@ -29,6 +28,8 @@ class DocsHelper:
         """
         Format the clock with engineering notation
         """
+        import matplotlib as mpl
+
         if clock == 0:
             return "No Clock"
         else: