Minor release with several bug fixes. Thanks to @kx-au for spotting and suggesting fixes to the bugs in the ImplicitLeapfrogIntegrator and solve_fixed_point_direct implementations. A previous attempt at adding a more helpful message when an error is encountered due to using parallel sampling with Autograd model functions without having multiprocess installed incorrectly assumed the exception raised would always be a PicklingError while in reality an AttributeError was being raised. The code now handles this case and correctly outputs an error message suggesting for the user to try installing multiprocess when trying to sample chains in parallel with Autograd model functions when multiprocess is not available. Several bugs / name errors in the implementation of the mici.integrators.ImplicitLeapfrogIntegrator have been fixed which previously prevented instantiation of this class. A bug in the naming of the tolerance parameter in mici.solvers.solve_fixed_point_direct introduced in 5e012034 has been fixed. The classes in the mici.matrices module have been refactored to use abc abstract base classes to make the expected interfaces for the various matrix types more explicit and docstrings added to most classes and methods. The mici.matrices unit tests have been updated to improve the coverage of the classes and reduce code duplication in writing the tests. The mici.integrator unit tests coverage has also been improved slightly by adding a basic test for a ImplicitLeapfrogIntegrator instance.