Vector form of affine system

[mforets]

This is a follow up of https://github.com/JuliaReach/MathematicalSystems.jl/pull/223

julia> b = rand(1); @system(x' = x + b)
AffineContinuousSystem{Float64,IdentityMultiple{Float64},Array{Float64,1}}([1.0], [0.4376218479701559])

julia> b = rand(10); @system(x' = x + b, dim=10)
ERROR: AssertionError: checksquare(A) == length(c)
Stacktrace:
 [1] AffineContinuousSystem(::IdentityMultiple{Float64}, ::Array{Float64,1}) at /home/mforets/.julia/dev/MathematicalSystems/src/systems.jl:189
 [2] top-level scope at REPL[4]:1

julia> b = rand(10); @system(x' = x + b)
ERROR: AssertionError: checksquare(A) == length(c)
Stacktrace:
 [1] AffineContinuousSystem(::IdentityMultiple{Float64}, ::Array{Float64,1}) at /home/mforets/.julia/dev/MathematicalSystems/src/systems.jl:189
 [2] top-level scope at REPL[5]:1