Issue better information messages when building the doc

Currently, when building the doc with Sphinx:

1. When main.R has not been run before compiling the doc, we get the following error message:

$HOME/matpopmod/doc/api/compadre.rst:11: WARNING: Problems with "include" directive path:
InputError: [Errno 2] No such file or directory: 'doc/compadre.inc'.
$HOME/matpopmod/src/matpopmod/compadre.py:docstring of matpopmod.compadre:37: WARNING: Undefined substitution referenced: "COMPADRE JSON".
$HOME/matpopmod/src/matpopmod/compadre.py:docstring of matpopmod.compadre:38: WARNING: Undefined substitution referenced: "COMADRE JSON".
$HOME/matpopmod/src/matpopmod/compadre.py:docstring of matpopmod.compadre:40: WARNING: Undefined substitution referenced: "LONG DATE CV".
$HOME/matpopmod/doc/quickstart.rst:326: WARNING: Problems with "include" directive path:
InputError: [Errno 2] No such file or directory: 'doc/compadre.inc'.
$HOME/matpopmod/doc/quickstart.rst:218: WARNING: Undefined substitution referenced: "COMPADRE JSON".
$HOME/matpopmod/doc/quickstart.rst:218: WARNING: Undefined substitution referenced: "SHORT DATE CV".
$HOME/matpopmod/doc/quickstart.rst:222: WARNING: Undefined substitution referenced: "COMADRE JSON".
$HOME/matpopmod/doc/quickstart.rst:222: WARNING: Undefined substitution referenced: "SHORT DATE CV".

It could be useful to indicate somewhere that compadre.inc is supposed to be generated by running main.R; and maybe to include a default compadre.inc to suppress these error messages (while still indicating that main.R should be run.

2. Generating some of the plots takes time and it sometimes looks like the build has frozen; it would be nice to display a little something saying "generating plot xxx, this may take some time -- done".