Standardise and document object modification methods like shiftlocus

[rafaqz]

shiftlocus should be listed here:
https://rafaqz.github.io/DimensionalData.jl/stable/object_modification

It should also have the same syntax as set and other object modification methods. Probably they all need some standardising

[rafaqz]

We may be able to roll the user facing-part of rebuild, set, shiftlocus and even reorder into a single function. This should fix everything to match what you specify.

We will then need another function tha updates traits/values without fixing anything else.

Maybe this is set and unsafe_set. I'm not sure. But set can replace rebuild by adding keywords. Then rebuild can be more of the internal thing.