Add general changes from volleyball project

[corneliusboehm]

As part of the volleyball example in https://github.com/corneliusboehm/sense/pull/5, some general changes were added, which would make sense to be contained in this repository's main branch.

Features

• DisplayCounts has been added as a generalized version of DisplayRepCounts, which doesn't show the Exercise: prefix and can highlight counts that have recently been updated.
• title in DisplayResults has been made optional
• The class AggregatedPostProcessors has been added, which takes a list of PostProcessors and stores their results together in the output directory under a given out_key. This class replaces the previous PostprocessRepCounts and PostprocessEventCounts.
• The class ExerciseSpecificRepCounter has been generalized to TwoPositionsCounter
• EventCounter can now take a custom out_key
• In get_relevant_weights you can now request a specific feature converter

Bug Fixes

• If the --path_in file didn't exist, cv2.VideoCapture was still created and did not throw an error except that the reported fps was 0. Now a FileNotFoundError will be raised.
• If the setup of the stack took a little longer (e.g. because a long video needed to be read and resampled), the displayed video and model fps would start very low and only slowly recover, because the first measured delta included the setup time. Now there is an initialize call on BaseDisplay which will be called when all setup is done.
• Add a check if temporal_annotation is empty