Add doctests for liberty_float edge case

Issue #280
diff --git a/scripts/python-skywater-pdk/skywater_pdk/liberty.py b/scripts/python-skywater-pdk/skywater_pdk/liberty.py
index b14ec3e..77f2bb5 100755
--- a/scripts/python-skywater-pdk/skywater_pdk/liberty.py
+++ b/scripts/python-skywater-pdk/skywater_pdk/liberty.py
@@ -769,6 +769,15 @@
     >>> liberty_float(1)
     '1.0000000000'
 
+    >>> liberty_float(1e9)
+    '1000000000.0'
+
+    >>> liberty_float(1e10)
+    '1.000000e+10'
+
+    >>> liberty_float(1e15)
+    '1.000000e+15'
+
     >>> liberty_float(True)
     Traceback (most recent call last):
         ...