Add an attribute for a point preceding a filter

[tomjholland]

This pull request includes several changes to the pyprobe codebase, focusing on adding new functionality for handling preceding points in data frames.

Handling Preceding Points:

• pyprobe/filters.py: Added preceding_points attribute to Procedure, Experiment, Cycle, and Step classes. [1] [2] [3] [4] [5]
• pyprobe/filters.py: Added _get_preceding_points static method to Procedure class to extract rows prior to the first point of each event.
• pyprobe/filters.py: Updated methods to include preceding_points in their parameters and initialization. [1] [2] [3]

Test Updates:

• tests/analysis/test_cycling.py: Updated Cycling_fixture to include preceding_points attribute.
• tests/test_procedure.py: Added tests for get_preceding_points method and updated existing tests to check for preceding_points attribute. [1] [2] [3]
• tests/test_rawdata.py: Updated RawData_fixture to include preceding_points attribute. [1] [2]