update pandas version to use diff

[veenstrajelmer]

Code would be slightly more convenient if we update to pandas 2.1.4, since it supports diff for timedeltas. That pandas version supports python 3.9 and higher, which is also what kenmerkendewaarden supports.

.diff() can be applied in two locations:

kenmerkendewaarden/kenmerkendewaarden/data_analysis.py

Lines 224 to 226 in 957c07f

	ts_timediff = (
	df_times[1:] - df_times[:-1]
	) # TODO: from pandas 2.1.4 the following also works: df_times.diff()[1:]

And:

kenmerkendewaarden/kenmerkendewaarden/tidalindicators.py

Lines 224 to 226 in 957c07f

	df_meas["timediff"] = pd.TimedeltaIndex([pd.NaT]).append(
	df_meas.index[1:] - df_meas.index[:-1]
	) # TODO: from pandas>=2.1.4 the following also works: df_times.diff() (which results in a timedeltaindex of the correct length)