ComputeGlobalsElement for the modular simulator

ComputeGlobalsElement for the modular simulator

This element encapsulates the calls to `compute_globals`. A new
approach to fit the global computation calls into the client approach has
been proposed in I44a7193c, but a complete rewriting is currently outside
the scope of this effort. This element therefore aims at offering an
interface to the legacy implementation which is compatible with the new
simulator approach.

The element comes in 3 (templated) flavors: the leap-frog case, the first
call during a velocity-verlet integrator, and the second call during a
velocity-verlet integrator.

This commit is part of the commit chain introducing the new modular
simulator. Please see docs/doxygen/lib/modularsimulator.md for details
on the chosen approach. As the elements of the new simulator cannot all
be introduced in one commit, it might be worth to view Iaae1e205 to see
a working prototype of the approach.

Change-Id: I35f310daed6355dc9ea6b574d855929ec2fee6fe