stdzing i/o for metab functions, especially BK

They will all have ... as an argument, and same order of arguments. All return GPP, R, NEP (possibly as part of list). BK no longer accepts latitude, date.time. Now, k.gas is supplied to BK as it is for other model (adjusted for size of time step), and irr is supplied as integers (0 for night, 1 for day). Plan to further refine output formats NOT in the metab.xx() functions, but in metab(). This will allow conquer.list() to conveniently stack multiple data.frames in the same metab.xx() output (see metab.kalman, e.g.), into a single many-row data.frame or matrix; this larger data.frame/ matrix can then be turned into an attribute in metab().