Document ways of accessing numerical data

[agcapps]

Add material to tutorial_cpp_numeric.rst that compares and contrasts three methods of access to numeric array data:

• direct pointer access
• DataArray
• DataAccessor

[cyrush]

This looks great!

The output of the docs is not automatically captured, so if we want to update the output to include the new example, we need to run the test and then pipe to https://github.com/LLNL/conduit/blob/develop/src/docs/sphinx/t_conduit_docs_tutorial_numeric_out.txt and commit the change.