Add missing sparse routines (sobj, sgrad)

[jfowkes]

The official CUTEst MATLAB interface has the sparse routines:

Tool    CUTEst function   Purpose
--------------------------------------------------------------------------
sobj    uofg / cofsg      Evaluate objective function value
                          and its sparse gradient if requested
sgrad   ugr / cisgr       Evaluate objective function or
                          constraint gradient as a sparse vector

These are the sparse equivalents of our existing obj and grad routines.

We should probably implement these in a similar way as done there:
https://github.com/ralna/CUTEst/blob/master/src/matlab/mcutest.c