Added back delete_recordings function + Moved find_char to process.utils + remove last mention of image_format and nb_leading_zeros in BatchProcessing

Removed all self.image_format and self.nb_leading_zeros (currently still some image_format and nb_leading_zeros to replace)

Changes to interp_2d_obj, tracker2d.load_images and added TODO to use ImageSequences and not use image_format or nb_leading_zero

- Load recordings from directories
- Use the names and paths of the recordings and images

BatchProcessing is now using MultiImageSequence to load and work with the recordings (sequences of images), various path variables have been changed

Added new test functions in test_process_batch.py and renamed test_process.py to test_process_utils.py

Added new test functions in test_process_batch.py and renamed test_process.py to test_process_utils.py

+ various small changes

Moved functions out of BatchProcessing into dlc_postprocessing.py, point_tracking.py and skeleton_fitting.py

Changed signature of many functions in process module to make the order mor intuitive, the name more clear and add default value to show_plot variable (=False)

Added camera directories (cam1, cam2 and cam3) for the example recordings + changed the name of 'mosquito_escape' folder to 'mosquito_escapes'

Added gen_dlt_coefs_and_save_csv in calib.py + Debugging and documenting calib.py, test_calib.py and reconstructor3d.py