Use functions from solver module in Diffusion class

[mdpiper]

In this PR, I modified the ivy-diffusion Diffusion class to use the functions from the solver module. This provides a nice way to show:

• relative imports
• using/reusing code from another module

In order to make relative imports work, I had to include a __init__.py file in the ivy-diffusion package (see #131). I had been hoping to avoid this extra step.

I also changed the way the time step is calculated in the solver module. It was way unstable otherwise.

This resolves #129.