Draft: Added "get_hist", "interpolate_closest_values", "get_voltammogram_group", "group_average" functions and corresponding tests

Added get_hist() and get_hist_group functions in seccm/postprocessing.py. These functions return the histogram of the distribution of Current Density (CD) values at a given voltage for all the repetitions of the measurements made. While get_hist() plots CD distributions from all CD-Voltage curves, get_hist_group() plots distribution from CD-Voltage curves belonging to a given overlap group.

Added interpolate_closest_values() function in seccm/utils.py. This function returns the CD values (from both increasing voltage cycle and decreasing voltage cycle) at a given voltage by interpolating the CD values at the neighbouring voltage values. Corresponding test for this function TestInterpolate has been added to tests/test_utils.py. This function is made use in the MeasurementSeries class in measurement.py module.

Added functions get_voltammogram_group() and get_voltammogram_group_at_voltage() in seccm/postprocessing.py. These functions return the CD-Voltage readings for measurements belonging to a given overlap group. While get_voltammogram_group() returns the complete CD-Voltage readings, get_voltammogroup_group_at_voltage() returns the CD values at a given voltage. Corresponding tests for both these functions are added in tests/test_measurement_postprocessing.py.

Added function group_average() in seccm/postprocessing.py This function takes the CD-voltage readings from all the measurements in a given overlap group and returns an averaged CD-voltage readings. No test for this function is added yet.

Linting was carried out but few style inconsistencies still remain.