Add resource levelling capability

[jakesee]

I don't really understand the logic for resource levelling, but someone has requested it, so I found this online as a start: https://www.mpug.com/articles/finally-the-resource-leveling-feature-explained-2/

[MannyPMP]

Resource level has to do with spreading the use of a given resource evenly for the duration of the project. You have to move tasks that use a given resource to level the resource. Critical path resource usage cannot be leveled.