Default max step-size for RF waveforms

[cncastillo]

For the time being, the max allowed RF step-size is Δt_rf=5e-5 or $50 \mu\mathrm{s}$. This is relevant to correctly simulate when we have considerable relaxation while the excitation is occurring. Regardless, this has not been thoroughly tested. Maybe $100 \mu\mathrm{s}$ is enough for most cases?

The effect this would have is to accelerate the simulation with simulate(...), without specifying simParams (default parameters).