NEW: Inertia type

[AlexS12]

At the moment, inertia is passed to state_equations as a SMatrix. However:

• the state equations assume that some terms of the matrix are zero and do not consider them even if they are not: It is considered that the aircraft xb-zb plane is a plane of symmetry so that Ixy and Iyz cross-product of inertia are zero and will not be taken into account.

  Ixx, Iyy, Izz = inertia[1, 1], inertia[2, 2], inertia[3, 3]
  Ixz = inertia[1, 3]

• the state equations use several products of terms of this matrix that are recomputed each time step

  IxzS = Ixz^2
  xpq = Ixz * (Ixx - Iyy + Izz)
  gam = Ixx * Izz - IxzS
  xqr = Izz * (Izz - Iyy) + IxzS
  zpq = (Ixx - Iyy) * Ixx + IxzS
  ypr = Izz - Ixx

Both reasons suggest that it might be a good idea to create a struct like:

struct Inertia{T}
    Ixx::T
    Iyy::T
    Izz:T
    Ixz::T
end

that could cache in its methods or store as fields IxzS, xpq, gam, xqr... and ensure that Ixz and Iyz are zero at construction time.