Convert between Workspace/Version/Microversion references

[harmanpa]

Some methods allow all WVM options, some are specific. Likewise responses might contain a specific option. For example, an assembly definition contains Microversion references for Parts, but for standard-content parts these might be Version references.

It would be useful for OnshapeDocument to have a method that returns an OnshapeDocument that includes the desired option. This might be just returning the same object, if the option required is included, or might require a call to getDocument to determine the option value.

(An additional point is that the name OnshapeDocument is incorrect, it should really be OnshapeElementReference or similar)