v0.23.0

Release 0.23.0

Breaking changes:
- refactoring of parsers, split into different modules in the new
  `stagpy.parsers` module. While technically a breaking change, this
  shouldn't affect most users since parsers are not meant to be called
  directly. Many edge cases were resolved, notably when hdf5 and legacy
  outputs are mixed.
- field variables are now defined differently in `phyvars`, leveraging
  the new `FieldVars` class. This change should also not affect most
  users.
- the `StagyyData.hdf5` property has been dropped, as well as
  `StagyyData.filename()`. Consider using the functions
  `StagyyData.par.h5_output()` or `StagyyData.par.legacy_output()` if
  necessary, although those should not be needed in user scripts.
- remove `rho4rhs` field as this isn't a standard output of StagYY and
  was the result of a temporary test.

Improvements:
- support `H_GPE` time series
- support `Pdyn` scalar field
- fix reading spherical geometry in HDF5 with parallel output