Petab

* Fixed various documentation issues
* Parameter mapping: Added option to ignore time-point specific
noiseParameters (51)

* Updated package URL
* Fixed noise formula check (49)
* Fixed override check and add noise formula check (48)
* Fixed timepoint override check (47)

Update python version for pypi deployment, no further changes

**NOTE:** The original PEtab format + petab package repository has been split
up (PEtab-dev/libpetab-python41). This repository now only contains the petab
Python package. The PEtab specifications and related information are available
at https://github.com/PEtab-dev/PEtab.

* Improved `petab.flatten_timepoint_specific_output_overrides` (PEtab-dev/libpetab-python42)
* Validator: output message in case of successful check is added (PEtab-dev/PEtab487)
* Update how-to-cite (Closes PEtab-dev/PEtab432) (PEtab-dev/PEtab509)
* Broadcast and mapping of scale and unscale functions (PEtab-dev/PEtab505)
* Update Python requirement (3.7.1) (PEtab-dev/PEtab502)
* Fix `petab.get_required_parameters_for_parameter_table`
(PEtab-dev/libpetab-python43)
* Fix `petab.measurement_table_has_timepoint_specific_mappings`
(PEtab-dev/libpetab-python44)

* Fix sampling of priors in `parameterScale` (PEtab-dev/PEtab492)
* Clarify documentation of `parameterScale` priors
* Improvements in `petab.simulate` (PEtab-dev/PEtab479):
* Fix default noise distributions
* Add option for non-negative synthetic data

* Fix for pandas 1.2.0 -- use `get_handle` instead of `get_filepath_or_buffer`
* Fix erroneous `petab_test_suite` symlink (all PEtab-dev/PEtab493)