fix bug calculate find_tps_tm when the end time fall into the last 5 minute of a day

closes #81 (closed)

Logic changes are only at the end of handling_data.py find_tps_tm_idx() to handle the case that x_idx is None.

Other changes are variables and functions name to be more descriptive and unittests are added to test the 2 functions involve in tps calculating: get_start_5mins_of_diff_days() and find_tps_tm_idx()