New plot: Signal vs diffusion time and q value

In order to see the behaviour of non Gaussian signal, we want to be able to visualise the signal as a function of q and Delta independent from the diffusion time.

This plot is designed to do that. It will be like the current plot_signal_vs_bvals. The only difference is:

• x-axis will be diffusion time
• two sliders added which manipulate G / delta