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.
This feels like it should be `add_items` rather than `add_fields`.  (I'm also having second thoughts about having renamed `minor_axis` to `sids`).