simplify LinearIndexer
add tentative weighted l1 norm
allow initial values for x and y in APGM
optimize APGM speed by using explicit LinearOperators for the identity operator
correctly estimate n in APGM for constructing default matrix B
new operator: L2Distance
new operator GroupL1Norm (which implements the mixed l1-l2 norm)
new example: Total Variation image denoising
merge and fix sp.eye()