clean up hatyan.calc_HWLW12345to12()

[veenstrajelmer]

Todo:

• with the current method there is no first LW computed even if the timeseries would start with 3/4/5, since that function starts with the first HW. This could be avoided by adding a dummy HW at the start of the timeseries, although in case of only 5 that might result in a wrong LW. >> leave this be
• there is an exception included to continue if the last HW is also the last value of the timeseries. This is not necessary since it is already catched by len(tide_afterHW)==0 so this can be removed >> this is an exception to ignore missing LW's, which is useful to keep
• code cleanups
• expand test coverage with specific testcases