Hydropandas

What's Changed
* Update to version 0.7.0 by OnnoEbbens in https://github.com/ArtesiaWater/hydropandas/pull/88
* some small fixes by OnnoEbbens in https://github.com/ArtesiaWater/hydropandas/pull/91


**Full Changelog**: https://github.com/ArtesiaWater/hydropandas/compare/v0.6.1...v0.7.0

What's Changed
* 79 add optional dependencies by martinvonk in https://github.com/ArtesiaWater/hydropandas/pull/80
* update to 0.6.1 by OnnoEbbens in https://github.com/ArtesiaWater/hydropandas/pull/81


**Full Changelog**: https://github.com/ArtesiaWater/hydropandas/compare/v0.6.0...v0.6.1

New features:
- get different types of evaporation from measurements (Pennman, Makkink or Hargraves)
- get an evaporation time series which is interpolated from the time series of multiple measurements stations
- write an ObsCollection to an .xlsx file with one overview tab and another tab per observation
- merge two observations and merge two observation collections.

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.

In version 0.5.0 the KNMIObs is removed and 3 new objects are created instead:

- MeteoObs for any type of meteorological observation
- PrecipitationObs a child class of MeteoObs for precipitation data
- EvaporationObs a child class of MeteoObs for evaporation data

The update also contains some modifications to the knmi module:
- missing precipitation data from a meteo station can now be filled with nearby data of a precipitation (neerslag) station.
- if you want to use the knmi api but the api is offline the non-api method for downloading data will be used.

Some small improvements:
- Remove pandas max dependency
- improve examples
- add interpolation to from_modflow method
- improve interpolation
- fix deprecation warning pyproj
- allow getting modellayer for piezometers from xarray DataSet