timestep_min None instead of np.nan

[veenstrajelmer]

Replace np.nan with None when reading timeseries in timeseries.py:

hatyan/hatyan/timeseries.py

Lines 1410 to 1412 in 42c9e23

	if len(data_meta_mincontent)==5: #nonequidistant timeseries
	timestep_value = np.nan #TODO: None is supported by pandas 2.1.2 and maybe earlier versions, but not 2.0.3 (py39 only)
	timestep_unit = np.nan

Altough, None in dataframes seems not supported by older pandas versions (ends up being nan anyway). So wait until support for Python 3.8 is dropped (#87)

Also update equidistant check in timeseries.py:

hatyan/hatyan/timeseries.py

Lines 1582 to 1583 in 42c9e23

	if np.isnan(diablocks_pd.loc[block_id,'timestep_min']):
	data_pd_oneblock = readts_dia_nonequidistant(filename_one, diablocks_pd, block_id)

Would avoid the need to overwrite nan with None in metadata.py:

hatyan/hatyan/metadata.py

Lines 43 to 45 in 42c9e23

	if np.isnan(metadata['timestep_min']): #non-equidistant, nan in py38 and none in py39 (pandas 2.1.2)
	metadata['timestep_min'] = None
	metadata['timestep_unit'] = None

[veenstrajelmer]

Since timestep_min is not added as metadata anymore since #313, this issue is not relevant anymore.