Incorrect objective calculation for SIRT

[MargaretDuff]

Description

In SIRT the objective calculation is here:

CIL/Wrappers/Python/cil/optimisation/algorithms/SIRT.py

Lines 203 to 209 in 9ba92fe

	def update_objective(self):
	r"""Returns the objective
	.. math:: \frac{1}{2}\|A x - b\|^{2}
	"""
	self.loss.append(0.5*self.r.squared_norm())

But $r\neq Ax-b$ because it is modified by the preconditioner:

CIL/Wrappers/Python/cil/optimisation/algorithms/SIRT.py

Line 193 in 9ba92fe

self.r *= self.M

Instead, we should have

    def update_objective(self):
        r"""Returns the objective

        .. math:: \frac{1}{2}\|A x - b\|^{2}

        """
        self.loss.append(0.5*(self.operator.direct(self.x)-self.data).squared_norm())

[MargaretDuff]

@epapoutsellis - if you have a moment, please can you sense check this for me?