Pyfstat

- 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

- PyPI source tarball for v1.11.4 didn't include files needed to build conda package
- improved codecov setup
- no actual changes to package

- python 3.9 now supported
- improvements to documentation and examples
- for developers: flake8-docstrings and flake8-executable rules now enforced
- added a codemeta.json file
- started tracking test coverage with codecov

- added LineWriter class for simulating unmodulated noise artifacts

- pinned numpy dependency to <1.20 to fix incompatibility with lalsuite 6.81
- updated ephemerides instructions and citation requests in README
- fixed FrequencyModulatedArtifactWriter class