ENH: Add Stairs.cummax #89
Description
Describe the solution you'd like
See analogous function in pandas https://pandas.pydata.org/docs/reference/api/pandas.Series.cummax.html
Requires skipna parameter. returns new Stairs instance with same initial_value and same value of "closed" property.
Functionality should be straightforward by applying pandas.Series.cummax to Stairs._data["value"] and using Stairs._new
The result will be a monotonically increasing step function
Additional context
https://www.staircase.dev/en/latest/development/guide/internals.html