Mt-metadata

---------------------

* minor bug fixes
* update reading .AVG files

--------------------

* Bug fixes (mainly in the transfer functions)
* Combined timeseries and transfer_function metadata where similar, so now transfer_function metadata imports from timeseries when applicable. Reduces files and redundancy.
* Updated documentation

--------------------

* Updating how transfer functions are handled
* Added readers and metadata standards for
- EDI files
- Z-files
- J-Files
- AVG file
- EMTF XML files
* Added tests for transfer functions
* Updated mt_metadata.transfer_functions.core.TF
* Added documentation on transfer functions

--------------------

* Bug fixes (mainly in mt_metadata.timeseries.filters)
* Updated tests
* Channel codes are handled "better"
* Updating translation between Experiment and StationXML
* Updated how filters are plotted
* Adding notebooks to documentation

-------------------

* Bug fixes
* Updated how units are handled
* If survey.id is None set it to fdsn.network if available (mainly an IRIS DMC fix)
* Updated translation of Experiment to StationXML
* Updated tests

-------------------

* Added time series filters

- PoleZeroFilter
- CoefficientFilter
- FIRFilter
- TimeDelayFilter
- FrequencyAmplitudePhaseFilter

* Added translations to/from StationXML
* Updated tests
* Fixed minor bugs
* Updated documentation