Allows for creation of groups with empty sets of providers
Splits out create_group and update_group into separate methods, instead of a single add_or_update_group method (NB: events remain the same, following the pattern established with other resources, e.g. providers)
Adds assert_valid_providers_vec validation method that panics if exact set of providers already exists in another group, and ignores if it is an empty set
create_groupandupdate_groupinto separate methods, instead of a singleadd_or_update_groupmethod (NB: events remain the same, following the pattern established with other resources, e.g. providers)assert_valid_providers_vecvalidation method that panics if exact set of providers already exists in another group, and ignores if it is an empty setResolves https://github.com/PotLock/core/issues/73 Resolves https://github.com/PotLock/core/issues/74