Pyfstat

- new dependencies: `attr` (core), `flaky` (for tests only)
- removed deprecated option to use `$LALPULSAR_DATADIR` for ephemerides
- added `SignalToNoiseRatio` class (equivalent to `lalapps_PredictFstat`)
and `DetectorStates` helper class in new `snr` module
- ` InjectionParametersGenerator` and its children moved
to separate module `injection_parameters`
- `Writer` (and its children) now accepts `timestamps`
as a list, dict over detectors,
or comma-separated string of files per detector
(`timestampsFiles` input is now deprecated)
- some simplifications to `Writer` internal methods
- for developers:
- test suite now split up by module,
full suite can now be run with `pytest tests/`
- flaky MCMC tests will be rerun 3 times if needed
- now enforcing `isort` import ordering style
and some other simple pre-commit-hook rules

- add new helper function `get_sft_as_arrays()`
- deprecate `get_sft_array()`
- the new one supports multiple IFOs
and returns full complex amplitude info
- note that the order of returned values is different

- now tested for python 3.10
- simpler handling of ephemerides files:
- lalsuite >= 7.2.0 (lalpulsar >= 3.1.1)
now includes a sufficient minimal set by default
and has gotten much better at resolving default paths
- no manual setup should be required any more for `pip` installs
- remove download script
- deprecate using `$LALPULSAR_DATADIR`
- bump LALSuite version requirements correspondingly
- `Writer` classes: fix bug with spurious expected filename mismatch
if given `noiseSFTs` including multiple frequency-segmented SFTs
- `GridSearch` now has `generate_loudest()` method like MCMC classes
- fix some deprecation warnings

- add backwards compatibility workaround
for lalapps 7.3.0 in `FrequencyModulatedArtifactWriter`
to fix running with conda dependencies

- drop python 3.6 support
- require LALSuite >= 7.1
- no longer pinning numpy
- resolved various deprecation warnings
- follow updated "narrowband" SFT file name convention
from lalapps_splitSFTs
- new `timestampsFiles` option for `Writer` classes
- new `allowedMismatchFromSFTLength` option
for core and MCMC classes
- new `singleFstats` option for `ComputeFstat` and derived classes
- new `randSeed` option for `ComputeFstat` and derived classes
for reproducible on-the-fly Gaussian noise generation
- fix data-from-disk reuse in `LineWriter`
and some internal cleanup to `Writer` and derived classes in general

- new reference paper for PyFstat: https://doi.org/10.21105/joss.03000
- all requirements are now handled through setup.py,
e.g. instead of `pip install -r requirements.txt`
and manually installing optional dependencies,
just use `pip install pyfstat[optionalpackage]`
- improved formatting of MCMC corner plots
- extended cumulative 2F plots for the transient case
- ComputeFstat.get_semicoherent_twoF() now returns its value
- minor internal cleanups
- test coverage improvements