Changes regarding the Sphinx plot directive

Fixed a bug in the Sphinx plot directive (.. plot:: path/to/plot_script.py)

where the source Python file was not being found relative to the directory of

the file containing the directive. In addition, its behavior was changed to

make it more streamlined with other Sphinx commands.

Documents that were using this feature may need to adjust the path argument

given to the plot directive. Two options are available:

1. Use absolute paths to find the file relative the ``plot_basedir`` (which

defaults to the directory where conf.py is).

2. Use relative paths and the file is found relative to the directory of the

file containing the directive.

Before this change, relative paths were resolved relative to the source

directory (where conf.py is) and absolute paths were pointing to files in the

host system.