Add support for using non-negative least-squares optimization

In `calc_plot_adj` and `ifunc.calc_coeffs` add a new argument `nnls`.
If set to True then the function `scipy.optimize.nnls` [1] will be called
instead of the normal SVD solver.  The default is False for backward
compatibility.

[1]: http://docs.scipy.org/doc/scipy/reference/generated/scipy.optimize.nnls.html