Fix photontof index in WPTOF calculation

[rantahar]

Check List

Before you submit your pull-request, please verify and check all below items

• You have only modified the minimum number of lines that are necessary for the update; you should never add/remove white spaces to other lines that are irrelevant to the desired change.
• You have run make pretty (requires astyle in the command line) under the src/ folder and formatted your C/C++/CUDA source codes before every commit; similarly, you should run python3 -m black *.py (pip install black first) to reformat all modified Python codes, or run mh_style --fix . (pip install miss-hit first) at the top-folder to format all MATLAB scripts.
• Add sufficient in-code comments following the doxygen C format
• In addition to source code changes, you should also update the documentation (README.md, mcx_utils.c and/or mcxlab.m) if any command line flag was added or changed.

If your commits included in this PR contain changes that did not follow the above guidelines, you are strongly recommended to create a clean patch using git rebase and git cherry-pick to prevent in-compliant history from appearing in the upstream code.

Moreover, you are highly recommended to

• Add a test in the mcx/test/testmcx.sh script, following existing examples, to test the newly added feature; or add a MATLAB script under mcxlab/examples to gives examples of the desired outputs
• MCX's simulation speed is currently limited by the number of GPU registers. In your change, please consider minimizing the use of registers by reusing existing ones. CUDA compilers may not be able to optimize register counts, thus require manual optimization.
• Please create a github Issue first with detailed descriptions of the problem, testing script and proposed fixes, and link the issue with this pull-request

Please copy/paste the corresponding Issue's URL after the below dash

• #244