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.
I think this can just be written as `_extract_values = np.vectorize(attrgetter('value')).  Though even that is super inefficient.  I'm surprised there isn't a pandas/numpy method for this.