Support scoped key-value tree transform rules
Resolve several TODOs about having the knowledge of the "applied-forces"
option section scattered in different places. It is now possible to
create a "scoped" key-value tree transform rule interface such that all
rules created through it target a specific subtree in the target tree.
This makes it possible to have initMdpOptions() and initMdpTransform()
operate on the same tree in electricfield.cpp, and have the higher-level
structure only declared in mdmodules.cpp. Remove the dependency on
MDModules from electric field tests, since it can now use these two
methods without knowing anything about the presence of the higher-level
"applied-forces" section.
Change-Id: I8107b10af3c9c602f40297a2279e3b0449e27e8d