Document and export `.validLHS()`?

[krivit]

At the moment, methods for set.vertex.attribute() for subclasses of network such as those used in networkLite and ergm.multi cannot use the in-place assignment semantics of the network itself because the function .validLHS() that checks if the argument is something that can be assigned to is not exported. We should probably do so.

@CarterButts , would that be OK?

[krivit]

Alternatively, it can be moved to statnet.common.