Fix force element
As FEP is currently turned off for the modular simulator, the force
element was using an empty ArrayRef as an argument for the lambda
vector in the call to the legacy do_force(). Further tests revealed,
however, that some force calculations use the lambda vector without
checking if FEP is needed. To avoid problems related to this, an
allocated array is used. Note that this change gets superseeded
by Iee12697a, so this is just a temporary fix.
Change-Id: Ibd09e83a8ee61f6da8c7f6ab8989777218407d94