Rename ecut attributes when unit has changed

This MR includes !1207 (merged) so should be reviewed/merged after that.

Many objects somehow juggle the fact that:

x = Thing(ecut=50)
x.ecut == 50 / Hartree

This caused a significant of amount of confusion for me (again), so now x.ecut_Ha will exist instead of x.ecut, and its name implies that of course a conversion has taken place.