add HWLW numbering testcase

[veenstrajelmer]

add 19y/multiyear HWLW numbering validation by:

• numbering extremes derived from waterlevels (not astro)
• compare with reference primary extremes from RWS (this dataset is not yet available)
• computing time/height difference, nans should not be present in the difference
• convert numbering_FEWS_PG.py to unittest
• figures of numbering_extremes.py were shifted since the introduction of timezones, why?