Allow TR num_local_datasets to be set to 0

[khurram-ghani]

Describe the bug
A derived rule should be able to override the num_local_datasets property of BatchTrustRegion and return 0. This use-case is when a rule does not use local-datasets with the regions, and instead manages only global datasets. Returning 0 from this property should mean there are "no local datasets", however this results in a divide-by-0 error in two places in the ask-tell-optimizer:

• trieste/trieste/ask_tell_optimization.py

  Line 300 in 7a595dc

  if num_new_points < 0 or num_new_points % num_local_datasets != 0:

• trieste/trieste/ask_tell_optimization.py

  Line 672 in 7a595dc

  if num_new_points < 0 or num_new_points % num_local_datasets != 0:

Trieste should probably explicitly support and document this use-case, and add tests for it.