Support configurable multiplicative factor in ThroughputModel

[osukhoroslov]

The effective activity throughput (used e.g. to compute activity remaining work) is computed as throughput * factor, where throughput is computed using the throughput sharing model.

By default factor=1. To model various phenomena (e.g. variation of throughput or dependence of throughput of activity size) it should be possible to override the factor per each activity using the provided implementation of ThrougputFactorFunction trait, which takes activity size (volume) and SimulationContext (to access time and RNG).

This should generalize and replace BWModel from dslab-storage.

Details of implementation in SimGrid:

• https://github.com/simgrid/simgrid/blob/c6f2914980450d19e7b47590a88698bb2b30e5c6/include/simgrid/kernel/resource/Action.hpp#L261
• https://github.com/simgrid/simgrid/blob/76ec29f832083ef95c56afbd1e11fc29856a0770/src/kernel/resource/Action.cpp#L99
• https://github.com/simgrid/simgrid/blob/2516bc5bf0760874f7ccd2b878657e6ab442649d/docs/source/tuto_disk/tuto_disk.cpp#L149
• https://github.com/simgrid/simgrid/blob/master/examples/cpp/network-factors/s4u-network-factors.cpp