js api missing xxBetween persistence methods

[ssalonen]

Openhab-js api is missing xxBetween methods, e.g. .averageBetween

See https://www.openhab.org/javadoc/latest/org/openhab/core/persistence/extensions/persistenceextensions

Your Environment

• Version used: (e.g., openHAB and JS Scripting add-on version)
• Environment name and version (e.g. Chrome 76, Java 8, Node.js 12.9, ...):
• Operating System and version (desktop or mobile, Windows 10, Raspbian Buster, ...):

[florian-h05]

Thanks for the hint, I‘ll add them.