Add AbstractRemap and IdentityRemap

[jb-mackay]

Adds an AbstractRemap type and an IdentityRemap. The abstract type will be useful for the coupler's handling of remapping as it can call generic remap operations. The IdentityRemap does nothing (remap! becomes a broadcast) and is useful for abstracting map construction when components share the same boundary space.

• [x] Code follows the style guidelines OR N/A.
• [x] Unit tests are included OR N/A.
• [x] Code is exercised in an integration test OR N/A.
• [x] Documentation has been added/updated OR N/A.