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/global_bitcell_array.py

@@ -5,16 +5,16 @@
 # (acting for and on behalf of Oklahoma State University)
 # All rights reserved.
 #
-import bitcell_base_array
+from .bitcell_base_array import bitcell_base_array
 from globals import OPTS
 from sram_factory import factory
-from vector import vector
+from base import vector
 import debug
 from numpy import cumsum
 from tech import layer_properties as layer_props
 
 
-class global_bitcell_array(bitcell_base_array.bitcell_base_array):
+class global_bitcell_array(bitcell_base_array):
     """
     Creates a global bitcell array.
     Rows is an integer number for all local arrays.