molpopgen/fwdpy11: Several "demes" model fixes

Most of the issues/PRs are collected under milestone 0.16.0 From the change log: Bug fixes An integer type in the infinitely-many sites mutation model was changed from unsigned to signed. This does not affect previous results because unsigned overflow doing the "right thing" ended up with final values being correct. PR {pr}766 Issue {issue}765 Fix a bug where stopping/restarting the evolution of demographic models at time points where a deme goes extinct. It is not possible that this bug affected results from earlier versions, as attempting to stop/start at these time points raised exceptions. Issue {issue}775 PR {pr}774 Fix bugs in C++ back-end for discrete demographic models. In some cases, we were using the wrong vector of deme sizes to update the model, leading to runtime exceptions. PR {pr}802 PR {pr}803 Fix error in demes models where "replacement" models had 1 generation of overlap between ancestral/derived demes. Issue {issue}814 PR {pr}815 {user}apragsdale {user}molpopgen Behavior changes If a demographic model is evolved, pickled, unpickled, and then used to evolve, it is now possible that exceptions will raise. This change is due to the fix for Issue {issue}775 introduced in PR {pr}774. See issue {issue}777 for more background. Mass migration events implemented via {func}fwdpy11.copy_individuals and {func}fwdpy11.move_individuals now occur after sampling within a generation. This change makes the timings consistent with all other events and also makes certain operations easier/feasible. {pr}809 Calling {func}fwdpy11.infinite_sites during a simulation now raises RuntimeError. {pr}820 {issue}769 Models imported from demes now start the forward-time portion of the model 1 (one) generation before the most ancient end time of an ancestral deme. {pr}818 {user}apragsdale {user}molpopgen New features Add {func}fwdpy11.DiploidPopulation.add_mutation. PR {pr}764 PR {pr}799 Add {class}fwdpy11.NewMutationData. PR {pr}764 Add __copy__ and __deepcopy__ to {class}fwdpy11.DiploidPopulation. PR {pr}770 Add __deepcopy__ to {class}fwdpy11.DiscreteDemography. PR {pr}773 C++ back-end A population can now be checked that it is- or is not- being simulated. PR {pr}762 {class}fwdpy11.discrete_demography.DiscreteDemography now stores the migration matrix as a stack-allocated object and not a unique_ptr. PR {pr}785 {issue}781 Build system All GCC builds and CI tests on Ubuntu + GCC now apply a much stricter set of compiler options. {pr}779 {issue}778 Dependencies Bump pillow version in doc/requirements.txt. {pr}763 {pr}811 Bump all lib dependencies and some doc dependencies. {pr}80