mesh bp: add version number to schema

[xjrc]

In order to support backwards compatibility as Blueprint evolves, the schema should be augmented to include version information.

[xjrc]

This issue will become crucial once #504 is resolved as the associated refactoring will break compatibility with Conduit-Blueprint meshes from versions before the change. In order to mitigate the cost of migrating, helper functions should be provided to users to upgrade their mesh to a more recent version of Conduit-Blueprint (e.g. in the case of #504, update their polygonal/polyhedral topologies to the new format).

[cyrush]

Pushing this to 0.7.0