BUG: wrong count of stations in preprocessing reporting

The number of stations per subset was always 1 to high.

Reason:

The following was executed:

df.loc["# Samples", set_name] = df.loc[:, set_name].sum()
df.loc["# Stations", set_name] = df.loc[:, set_name].count()

But the .count() also counts the entry for # Samples and therefore the number of stations is 1 to high. Fix this with the following.

df.loc["# Samples", set_name] = df.loc[:, set_name].sum()
assert len(data) == df.loc[:, set_name].count()-1
df.loc["# Stations", set_name] = len(data)

The 2nd line is not required but ensures that the length of the data collection is equal to the number of stations.