Explicit time as candidate property

[rafaelab]

New explicit time feature needed for transient and other time-dependent studies.
This can probably be achieved with the current structure of the code, by using non-string properties, e.g., setProperty("time", t).

[lukasmerten]

When I remember correctly we are not planning to implement something in this direction in the 3.2 release. Therefore, I change the milestone.

[JulienDoerner]

The first step has been included with PR #525.