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.
My inclination would still be to put this on a normal method rather than the magic `__len__`.  One worry I might have is that it could be ambiguous from a user perspective whether `len` refers to the window length or the length of the actual buffer.