Create separate splitter workflow to specify a number of enumerators

[spwoodcock]

Is your feature request related to a problem? Please describe.

• Current workflow is based of average number of buildings required.
• Sometimes the team has a number of enumerators / mappers, and would like a number of tasks to match that.

Describe the solution you'd like

• We should have a separate workflow, that is wrapped in the same split_by_sql function but with a different param.
• The param should be num_of_enumerators.
• If specified, the SQL will ensure we get exactly that number of tasks, then split up the tasks based on an approximate even distribution of features, also factoring the roads/rivers as usual.

Additional context

• To facilitate this we probably need modular functions.
• We should have some common logic defined in one place, that is reused across multiple workflows:
  • num buildings
  • num enumerators