minor l-curve comments

1. estimate='CSD'is a parameter of L_curve, it is not used anywhere.
2. L_curve does not return anything despite the statement in the docstrings.
3. decimal=3 in prints, skcsd uses source widths of the order of tens of microns.
4. Rs is converted to an array, lambdas isn't, it would be quite useful, if it was.
