feat: add NoIntegrator meaning no definition
feat: add NoIntegrator meaning no definition

Some of the simulators, such as SteepestDescentSimulator, does not have
any integrator because they does not do time integration. For those
cases, define `NoIntegrator` struct and throw if NoIntegrator would be
read.

ToruNiina/Mjolnir,ToruNiina/Mjolnir,ToruNiina/Mjolnir