fermi finding algorithm changed with donor ionization
Ef attribute added to Structure
ignore overflow error added
Silicon added
M added in material attribute

M: number of equivalent valleys in the conduction band added
Revert "M added in material attribute"

This reverts commit bcb4d5c66d915a01cc133467ebd88d0828912ce8.
Si properties updated
maxiter added in fermi function
Ea replaced with Ei

ionization energies designation changed
diff2 added

discrete form of second order derivative d(a*df)  with non uniform mesh
non unifirm mesh added