Update to version 0.5.1

Some unused modules are removed together with their data and tests. These modules are:

• io_arctic
• io_pystore

Now, the recommended way to store and load an ObsCollection uses pickles:
ObsCollection.to_pickle('oc.pklz')

Reading can be done using:

import pandas as pd
oc = pd.read_pickle('oc.pklz')

The knmi module is modified to distinguish between precipitation and meteo stations as described in #70.