Simple class for strictly proper SISO linear systems
the main.cpp file demonstrates a system approximating a pure differentiator with a rolloff at 20rad/sec
-Intended for non-uniform time steps
-LTI::SisoSystem objects can be multiplied and divided in the usual way
-Simulates the system between time steps using a sparse matrix multiplication on a state space realization
-Assumes first order hold between input updates
Updates state with time-input pairs (t_i,u_i)
Output can be requested at any time (t_j,y_j)