Added physical constants and electrostatics module

Initial commit of constants.py
Initial commit of potentials/electrostatics.py
Updated imports to reflect new package name

Removed references to Surface_Reaction_Tools from:

adsorption/UnitTests/test_AVF.py
adsorption/UnitTests/test_theoretical.py
adsorption/adsorption_fitting_tools.py
adsorption/theoretical.py
potentials/electrostatics.py