Implement finite difference gradient for sparse matrices.
Document.
Synchronize.
Prepare for roboptim-core v2.0 release.
Prepare release v2.0.
Fix library and plug-ins VERSION/SOVERSION.

Backport patch from Debian package (2.7.0-1).
Synchronize.
Improve Gnuplot Jacobian display.
Implement cached Jacobian (when impl_jacobian is overriden).
Set CachedFunction parameters as protected.

The user may want to handle the different caches differently.