add framework for minimization of p-layer
add framework for minimization of p-layer

don't implement it yet, we need to think this over some more.

nickvandewiele/RMG-Py,pierrelb/RMG-Py,nyee/RMG-Py,nyee/RMG-Py,nickvandewiele/RMG-Py,chatelak/RMG-Py,pierrelb/RMG-Py,chatelak/RMG-Py