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.
It seems like knowing the context in which the panel is being created shouldn't be in the purview of this function.  Is there a way to make it so that this is always just passed the dt of the next bar to be inserted?