Remove early stopping from the Gradient descent algorithm

[MargaretDuff]

Description

We should remove this and encourage users to use a callback instead:

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

Lines 43 to 50 in b91c0eb

	rtol: positive float, default 1e-5
	optional parameter defining the relative tolerance comparing the current objective function to 0, default 1e-5, see numpy.isclose
	atol: positive float, default 1e-8
	optional parameter defining the absolute tolerance comparing the current objective function to 0, default 1e-8, see numpy.isclose
	"""
	def __init__(self, initial=None, objective_function=None, step_size=None, rtol=1e-5, atol=1e-8, preconditioner=None, **kwargs):

Environment

import cil, sys
print(cil.version.version, cil.version.commit_hash, sys.version, sys.platform)