-ocean_obm_deltaT: use standard naming

[matthiasmengel]

I see no difference to other ocean delta_T modifiers. So we can use standard terminology, right?
There may be the same problem as describe in issue #2.

[talbrecht]

Same issue as for POGivenTH, order matters here. Is the input temperature field modified or the calculated boundary temperatures? Therefore in pism-pik06 we introduced an additional modifier flag (which should be avoided).