Created by: pshriwise

This PR moves OpenMC's plotting capability to C++.

Right now the implementation is nearly the same as it was in Fortran (with a few small refactors along the way). I'm planning on doing some re-design after this PR goes in, so I'll ask that some of the translated code blocks (particularly in the ObjectPlot constructor) are overlooked unless reviewers feel strongly otherwise.

Several changes were needed in other locations to remove the plot.F90 and plot_header.F90 files entirely. This includes:

• moving word_count to string_functions
• adding a get_child_nodes to the xml_interface
• adding a time_stamp function to output
• moving index_cmfd_mesh to settings