Update test plots

• Update GVEC_Reader interface.
• Use numerical domain.
• Ignore nan when using max and min functions for plotting.
• Outsource error computation.
• Update plot title.
  • Note: Range of relative error is hardcoded into the plot.
• Display plotting grid and element boundaries.
• Fix issues with colorbar update.
• Move ParaView functions to diagnostics folder.
• Wireframe for ParaView test.
• Write ParaView data with time steps.