d3-color : added piecewise method

[Ledragon]

#25582

Please fill in this template.

• Use a meaningful title for the pull request. Include the name of the package modified.
• Test the change in your own code. (Compile and run.)
• Add or edit tests to reflect the change. (Run with npm test.)
• Follow the advice from the readme.
• Avoid common mistakes.
• Run npm run lint package-name (or tsc if no tslint.json is present).

If changing an existing definition:

• Provide a URL to documentation or source code which provides context for the suggested changes: 1.2.0
• Increase the version number in the header if appropriate.
• If you are making substantial changes, consider adding a tslint.json containing { "extends": "dtslint/dt.json" }.

[typescript-bot]

@Ledragon Thank you for submitting this PR!

🔔 @tomwanzek @gustavderdrache @borisyankov - please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

If no reviewer appears after a week, a DefinitelyTyped maintainer will review the PR instead.

[typescript-bot]

@Ledragon The Travis CI build failed! Please review the logs for more information.

Once you've pushed the fixes, the build will automatically re-run. Thanks!

[typescript-bot]

After 5 days, no one has reviewed the PR 😞. A maintainer will be reviewing the PR in the next few days and will either merge it or request revisions. Thank you for your patience!

[Ledragon]

@tomwanzek @denisname With this closed I think the global bundle can be bumped to 5.2.0. Shall I go fot this? Is there a way to bind the bundle to specific modules version?