Identity Operator - Domain and Range geometry should be the same

[MargaretDuff]

Description

In the Identity Operator, domain and range geometry can be set to be different:

CIL/Wrappers/Python/cil/optimisation/operators/IdentityOperator.py

Lines 40 to 47 in 5d569dc

	def __init__(self, domain_geometry, range_geometry=None):
	if range_geometry is None:
	range_geometry = domain_geometry
	super(IdentityOperator, self).__init__(domain_geometry=domain_geometry,
	range_geometry=range_geometry)

However, the forward and adjoint operators just return their input. They do not use the range geometry:

CIL/Wrappers/Python/cil/optimisation/operators/IdentityOperator.py

Lines 49 to 71 in 5d569dc

	def direct(self,x,out=None):
	r'''Returns the input data :math:`x`
	Parameters
	----------
	x : DataContainer or BlockDataContainer
	Input data
	out : DataContainer or BlockDataContainer, optional
	If out is not None the output of the Operator will be filled in out, otherwise a new object is instantiated and returned. The default is None.
	Returns
	-------
	DataContainer or BlockDataContainer
	:math:`\mathrm{Id}(x) = x`
	'''
	if out is None:
	return x.copy()
	else:
	out.fill(x)
	return out

Propose that the option to pass a range geometry is removed.

Discussed with @bosschmidt at the CIL user meeting hackathon.