Merge databased steps model into main

Add new DatabasedStepsModel as implementation of MainModel. This model allows to reconstruct the movement behaviour of agents from a trajectory file. If the input to 'trajectoryFileOrFolder' is a .traj file, the DatabasedStepsModel reconstructs the movement from the file. If the input to 'trajectoryFileOrFolder' is a folder, the DatabasedStepsModel creates a hash that is unique for all attributes that affect locomotion, including the simulation seed. If there already exists a trajectory file with the hash in the given folder, the DatabasedStepsModel reconstructs the movement from the file. If it doesn't, the DatabasedStepsModel uses the fallbackMainModel given in the attributes of the DatabasedStepsModel to compute new trajectories and save the corresponding .traj file with the hash in the given 'trajectoryFileOrFolder'.