dynamic history

Previously, all specs had to be pre-allocated by using the 'add_history'
function. This is now no longer required and instead serves as a hint to
the HistoryContainer to pre-allocate the space for the given spec.

History can grow by increasing the length for a frequency, adding a
frequency, or adding a field. It can grow with any combination of
these.
Not sure how I feel about putting a `len` on `RollingPanel`.  For comparison, the base pandas `Panel` has no `len`, presumably because it's not obvious what axis the length should refer to.