Add POIMultiTask, allowing to pass multiple unblinded and froozen parameter flags.

dhi/tasks/combine.py

dhi/tasks/exclusion.py

dhi/tasks/gof.py

dhi/tasks/likelihoods.py

dhi/tasks/limits.py

dhi/tasks/pulls_impacts.py

dhi/tasks/significances.py

docs/content/snippets/parameters.md

docs/content/snippets/plotexclusionandbestfit_param_tab.md

.../content/snippets/plotmultiplegoodnessoffits_param_tab.md

.../snippets/plotmultiplelikelihoodscansbymodel_param_tab.md

...ntent/snippets/plotmultiplesignificancescans_param_tab.md

docs/content/snippets/plotmultipleupperlimits_param_tab.md

docs/content/snippets/plotupperlimitsatpoint_param_tab.md