MAINT: Add dcgain method to State and Transfer

[ilayn]

Not extremely robust evaluation of low frequency value of dynamical models.

Main issue is the integrator/adder cancellations such as Transfer([1, 0], [1, 1, 0]) but direct evaluation at s=0 would give 0/0though having a finite DC gain.

Closes #81

[ilayn]

control's implementation is a bit too naive since everything returns inf if A is singular, say if you have the State representation of [1/(s+1), 1/s]. DC Gain should return 1 and inf but instate version A is singular so it returns [inf inf], But I already fixed the DC gain code anyways.

It turns out that my library design was quite bad and lead to circular imports throughout. Hence the delay.