Adds control/lti models

Adds system models for linear time invariant state-space descriptions to control module.

New Features

• [control]:
  • Added StateSpaceModel
  • Added Zero-order hold discretizer for state-space parametrizations
• [core]: Traits<>::zero() now support a simple heuristic for dynamically-sized Eigen matrices

Bug Fixes

• [doxygen]: Now generates documentation of Signal correct. Forward declarations need to be excluded
• [control]: Removed unnecessary self-assignment check
• [analysis]: Fixed missing explicit cast for template type deduction in TaskSpace