Allow accessing assets with write access without incrementing their version

[RiscadoA]

Problem

For the #270 issue, @roby2014 came upon the problem that every write access the version of the asset was being increased, even though it wasn't being modified.
This is a nuisance in this case, since a non-const reference was necessary but we knew we weren't modifying it anyway.

Solution

It would be nice to be able to specify that write shouldn't increase the version and then be able to increase the version manually later.