Use packages for imports.

Must set PYTHONPATH to include OPENRAM_HOME now.
Reorganizes subdirs as packages.
Rewrites unit tests to use packages.
Update README.md with instructions, dependencies etc.
Update sky130 module imports.
Change tech specific package from modules to custom.

compiler/modules/column_decoder.py

@@ -5,16 +5,16 @@
 #
 from tech import drc
 import debug
-import design
+from base import design
 import math
 from sram_factory import factory
-from vector import vector
+from base import vector
 from globals import OPTS
 from tech import cell_properties
 from tech import layer_properties as layer_props
 
 
-class column_decoder(design.design):
+class column_decoder(design):
     """
     Create the column mux decoder.
     """