Feature/metadata improvements

[Thomas-Z]

These changes are related to the addition of dimension information in the zcollection metadata.
The motivations behind these changes are as follows:

Prevent the creation of unreadable zcollections when adding a new partition with dimensions different from the original one.
Improve validation of inserted data.
Clean up some code that templated existing variables to obtain dimension sizes.
Enable new features made possible by this additional metadata.

These changes will require existing collections to be updated before new data can be written.

The following elements are implemented in this initial commit:

• Additional information in the collection metadata file:
  • zcollection version number
  • Known dimension sizes and chunking
• Data validation on insert:
  • Checks on dimension properties
  • Checks on variable data types and fill values
• Support for dropping immutable variables
• Addition of a Collection.add_dimension method
• Update of the update_deprecated_collection function
• Handling of non-updated read-only collections

The following tasks remain to be done:

• Complete the list of changes
• Add tests:
  • For Collection.add_dimension
  • Reading subsets of view variables (only reference, only view, mix)
  • Adding immutable variables
  • Removing immutable variables
• Restrict the ability to update an immutable variable
• Restrict the ability to add immutable variables to a view
• Finalize and fully test the deprecated collection and view update functions