Doc: added description of aperture FLUXERR in photometry section.

Doc: added a first batch of model-fitting parameters to the list of measurements.

Doc: Added :param: role for SExtractor measurements parameters and modified custom CSS to manage the :param: role.
Doc: Added targetparam class and setup targets in preliminary measurement parameter list.

Doc: Customized CSS to have equation permalinks visible when hovering the mouse.

Doc: removed non-compliant characters in citation labels that would create problems with Sphinx v1.6+ (see https://github.com/sphinx-doc/sphinx/issues/3917).