make single public function for slotgemiddelden

[veenstrajelmer]

• [x] make single public function for slotgemiddelden kw.calc_slotgemiddelden() that combines deriving yearly means and fitting linear model
• [x] make valid filtering for tidalindicators optional (requires argument min_count)
• [x] make clipping on physical break optional (requires argument clip_physical_break)
• [x] for extremes and waterlevels
• [x] this cleans up the user code for slotgemiddelden
• [x] make sure we at least get a single value of prediction for year_slotgem, maybe more years as happened before but this requires tstop_dt_naive
• [x] consider to increase test coverage or remove kw.slotgemiddelden.broken_linear_model