Accumulator - Nonuniform Time Step Size and Accumulation Cycle Definition

[rljacob]

(copied from MCT's old bugzilla)

from @jwalterlarson
Currently the Accumulator works on the assumption of uniform step size in the
accumulation cycle. This has the advantage of supporting a large share of what
is out there. Furthermore, it does this without introducing extra floating
point uncertainty (i.e. dt) in the result.

But, we should start confronting the idea that the averaging cycle might be best
defined in terms of a floating point value (like an hour) and also that we may
wish to start supporting nonumiform step sizes