added a new powerlaw model with a test

fix models reference in test and use super to init

needed to add powerlaw to __all__

need to use parameters.Parameter

power law model only takes 2 pars

tests still fail, but the error no longer makes any sense

fixed the tests.  Sigh.

skip test if no scipy

use the derivative!

added a new powerlaw model with a test

fix models reference in test and use super to init

needed to add powerlaw to __all__

need to use parameters.Parameter

power law model only takes 2 pars

tests still fail, but the error no longer makes any sense

fixed the tests.  Sigh.

skip test if no scipy

use the derivative!

corrected deriv function (was correct at some time earlier; must have
forgotten to pull...)