Dolci/fix interpolate warnings

[Ig-dolci]

Gusto has several warnings related to the interpolate usage. See:

The use of `interpolate` to perform the numerical interpolation is deprecated.
  This feature will be removed very shortly.
  
  Instead, import `interpolate` from the `firedrake.__future__` module to update
  the interpolation's behaviour to return the symbolic `ufl.Interpolate` object associated
  with this interpolation.
  
  You can then assemble the resulting object to get the interpolated quantity
  of interest. For example,
  
  from firedrake.__future__ import interpolate
  ...
  
  assemble(interpolate(expr, V))

This PR seeks to fix such warnings.

[jshipton]

"several" warnings? There were loads! Thanks so much for getting rid of them - this looks great!

[tommbendall]

Thanks very much for doing this! If I've understood, I spotted one that was maybe missed (which probably isn't picked up in our testing...)

[tommbendall]

Thanks very much for this!