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 have named this to `shift_digest`, as it shift the digest panels by one bar. I have also written a docstring for the HistoryContainer explaining the reason for the various parameters, including the newly added one.