Add Lp-norms as errors

[MichaelClerx]

Now have MSE and RMSE, but not L2-norm

class L2Error(pints.ProblemErrorMeasure):
    def __call__(self, x):
        return np.sqrt(np.sum(np.sum((
            self._problem.evaluate(x) - self._values)**2, axis=0), axis=0))

or general version

class LpError(pints.ProblemErrorMeasure):
    def __call__(self, x):
        return np.sum(np.sum((
            self._problem.evaluate(x) - self._values)**p, axis=0), axis=0)**(1/p)

or inf version

class LInfError(pints.ProblemErrorMeasure):
    def __call__(self, x):
        return np.max(np.abs(self._problem.evaluate(x) - self._values))

[MichaelClerx]

Not sure that double sum couldn't be made into a single one without the axis keyword, tbh. Should have a look at MSE/RMSE implementation that I copied that from