Standardise core code comments

[harshkhandeparkar]

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This is a sub-issue of #1046

The comments in the following files are inconsistent:

1. ImageSequencer.js
2. PixelManipulation.js

Rules:

1. Methods must be described using JSDoc comments
2. Misc code comments should be inline unless it is a long sentence.
3. No use of continuous tenses, no pronouns.
4. No redundant comments.
5. Each comment should start with an uppercase letter and end with a full stop.

[harshkhandeparkar]

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