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.
sounds good, I just made a window_length property, this lets people inspect this value without worrying about mutation. I also renamed window to `_window` to, The proper way to adjust the window would be through a call to resize, so this avoids confusion.