Refined DF Calculation in DDP for more general Expression for addressing variable length stages

[sambuddhac]

Description

This PR refines the evaluation of discount factor (DF) calculation in DDP making it more generic for variable length stages

What type of PR is this? (check all applicable)

• [x] Bug Fix
• [x] Documentation Update

Related Tickets & Documents

Fixes Issue #671 https://docs.github.com/en/free-pro-team@latest/github/managing-your-work-on-github/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword

Checklist

• [x] Code changes are sufficiently documented; i.e. new functions contain docstrings and .md files under /docs/src have been updated if necessary.
• [x] The latest changes on the target branch have been incorporated, so that any conflicts are taken care of before merging. This can be accomplished either by merging in the target branch (e.g. 'git merge develop') or by rebasing on top of the target branch (e.g. 'git rebase develop'). Please do not hesitate to reach out to the GenX development team if you need help with this.
• [x] Code has been tested to ensure all functionality works as intended.
• [x] CHANGELOG.md has been updated (if this is a 'notable' change).
• [x] I consent to the release of this PR's code under the GNU General Public license.

How this can be tested

Tested it against 6_three_zones_with_multistage

Post-approval checklist for GenX core developers

After the PR is approved

• [x] Check that the latest changes on the target branch are incorporated, either via merge or rebase
• [ ] Remember to squash and merge if incorporating into develop

[sambuddhac]

Thank you @sambuddhac! Approved it with some comments.

Thanks, @lbonaldo . I have incorporated all your suggestions. And have made sure to check the integrity of the code. Now, merging this to release branch.