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/local_bitcell_array.py

@@ -5,15 +5,15 @@
 # (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 tech import layer_properties as layer_props
 
 
-class local_bitcell_array(bitcell_base_array.bitcell_base_array):
+class local_bitcell_array(bitcell_base_array):
     """
     A local bitcell array is a bitcell array with a wordline driver.
     This can either be a single aray on its own if there is no hierarchical WL