Ismn

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- Added `ismn collect_metadata` and `ismn export_geojson` CLI programs.
- Added method to NetworkCollection to export metadata as (geo)json.
- Added more options when plotting the station overview map.
- Network citation list updated.
- Faster ISMN_Interface generation for large datasets by skipping some checks

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- Fixed bug with parsing sensor files for RISMA network in "header & files" format

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- Continue metadata collection for station on any type of error (PR 64)

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- Fix bug where station names in metadata can be different between Header and CEOP format.
- Custom Sensor Metadata reader now also checks the measured variable.

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- Added functionality to provide fill values with predefined custom metadata readers.
- Documentation and constants updated.
- ``ISMN_Interface.read_ts`` raised an error when np.int64 was passed.
- Fixed for `pygeogrids>=0.4.2` where dist=np.inf can be returned if

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- Add module to assign custom metadata readers to ISMN_Interface
- Notebook added that describes using a custom metadata reader
- RTD build uses a separate, smaller environment.yml now (and mamba)
- ISMN_Interface now has a method to create an instance of itself for a selection of (`ISMN_Interface.subset_from_ids`)