Fixes two bugs in dist_grid

Summary

Fixes two bugs in dist_grid routine yac_dist_grid_pair_get_vertex_neighbours, which lead to faulty results for creep and memory issues:

• Evaluation of vertex field mask was wrong; in case no mask was provided, no neighbours were found
• Maximum number of neighbours per vertex was assumed to be equal to the number of associated cells, which is true for inner vertices. However, vertices on grid boundaries can have an additional neighbour vertex.

Checklist

• All code changes are covered by unit tests
• Code coverage did not deteriorate
• Code follows Style Guide
• Documentation updated, if necessary
• WIP: Manual running of configure/build jobs was successful
• Update changelog, if changes are worth mentioning (Develop section in doc/src/changelog.dox, add section if not yet included)