Add checks in SemiImplicitEuler

[fabinsch]

check if it is second-order dynamics @quentinll

for example this test here fails:

ode = create_linear_ode(3, 2)
dt = 0.1
dyn = dynamics.IntegratorSemiImplEuler(ode, dt)
assert isinstance(dyn.createData(), dynamics.ExplicitIntegratorData)
ode_int_run(ode, dyn)

while for create_linear_ode(4, 2) it passes. see full test function here: https://github.com/Simple-Robotics/proxddp/blob/main/tests/python/test_integrators.py

[quentinll]

For the moment, IntegratorSemiImplEuler only works for systems whose state dimension is a pair integer (we make the hypothesis that x = [q,v] where dim(q)=dim(v), which is quite restrictive ...). This explains why this test fails.

[fabinsch]

maybe we can do the following for now ? #21

it is an assert. so it will not trigger when compiled in RELEASE mode or with the NDEBUG flag.
If it triggers in debug mode, it looks like the following

python: /home/fschramm/workspace/proxddp/include/proxddp/modelling/dynamics/integrator-semi-euler.hxx:12: proxddp::dynamics::IntegratorSemiImplEulerTpl<Scalar>::IntegratorSemiImplEulerTpl(const std::shared_ptr<proxddp::dynamics::ODEAbstractTpl<_Scalar> >&, proxddp::dynamics::IntegratorSemiImplEulerTpl<Scalar>::Scalar) [with _Scalar = double; proxddp::dynamics::IntegratorSemiImplEulerTpl<Scalar>::Scalar = double]: Assertion `((this->ndx1) % 2 == 0) && "IntegratorSemiImplEuler must be used with even ndx."' failed.
[1]    346945 abort      python test_integrators.py

which i think is understandable ? what do you think @quentinll

[quentinll]

This looks fine to me @fabinsch
For later, it would be better to add a class of second order dynamics. This class would be enforced for this integrator and it would provide the dim(q) and dim(v) informations. This would make it possible to consider systems for which dim(q) != dim(v).