opFunction Wrapper for functions.
opFunction(M,N,FUN,CFLAG,LINFLAG) creates a wrapper for function
FUN, which corresponds to an M-by-N operator. The FUN parameter
can be one of two types:
1) A handle to a function of the form FUN(X,MODE), where the
operator is applied to X when MODE = 1, and the transpose is
applied when MODE = 2;
2) A cell array of two function handles: {FUN,FUN_TRANSPOSE},
each of which requires only one parameter, X.
Optional arguments CFLAG and LINFLAG indicate whether the
function implements a complex or real operator and whether it
is linear or not. The default values are CFLAG=0, LINFLAG=1.