Very minor bugfixes. Documentation updates

Bugfixes

• [Python] Adding VectorPartitioned.copy*.
• is_unique gave a broadcast error, instead of returning false for non-unique data.

Documentation

• Removing ParaView example. Considered out-of-scope (better suited as example upstream).
• Examples now Python-only for readability.
• Using "furo" sphinx theme

Internal

• Removing GMat tests. It was not clear which purpose they served.