ApproximateGradientSumFunction class and example SGFunction for Stochastic Gradient Descent - #1550

Describe your changes

Added the ApproximateGradientSumFunction class and example SGFunction for Stochastic Gradient Descent and relevant unit tests

Describe any testing you have performed

See code

Link relevant issues

• Closes Implement SubsetSumFunction #1341
• Closes Stochastic Gradient Descent (SGD) #1343
• Supersedes ApproximateGradientSumFunction and SGFunction #1345

Checklist when you are ready to request a review

• I have performed a self-review of my code
• I have added docstrings in line with the guidance in the developer guide
• I have implemented unit tests that cover any new or modified functionality
• CHANGELOG.md has been updated with any functionality change
• Request review from all relevant developers
• Change pull request label to 'Waiting for review'

Contribution Notes

Please read and adhere to the developer guide and local patterns and conventions.

• The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in CIL (the Work) under the terms and conditions of the Apache-2.0 License.
• I confirm that the contribution does not violate any intellectual property rights of third parties