Fix mountain test through specification of initial wind

[tommbendall]

The mountain_nonhydrostatic test is currently broken, as it calls the remove_initial_w routine which no longer works, giving the error message:

Traceback (most recent call last):
  File "/home/thomas/firedrake/src/gusto/case_studies/case_studies/compressible_euler/mountain_nonhydrostatic.py", line 283, in <module>
    mountain_nonhydrostatic(**vars(args))
  File "/home/thomas/firedrake/src/gusto/case_studies/case_studies/compressible_euler/mountain_nonhydrostatic.py", line 224, in mountain_nonhydrostatic
    remove_initial_w(u0)
  File "/home/thomas/firedrake/src/gusto/gusto/initialisation/hydrostatic_initialisation.py", line 232, in remove_initial_w
    bc.apply(u)
  File "petsc4py/PETSc/Log.pyx", line 188, in petsc4py.PETSc.Log.EventDecorator.decorator.wrapped_func
  File "petsc4py/PETSc/Log.pyx", line 189, in petsc4py.PETSc.Log.EventDecorator.decorator.wrapped_func
  File "/home/thomas/firedrake/src/firedrake/firedrake/adjoint_utils/dirichletbc.py", line 32, in wrapper
    ret = apply(self, *args, **kwargs)
  File "/home/thomas/firedrake/src/firedrake/firedrake/bcs.py", line 448, in apply
    r = r.sub(idx)
  File "petsc4py/PETSc/Log.pyx", line 188, in petsc4py.PETSc.Log.EventDecorator.decorator.wrapped_func
  File "petsc4py/PETSc/Log.pyx", line 189, in petsc4py.PETSc.Log.EventDecorator.decorator.wrapped_func
  File "/home/thomas/firedrake/src/firedrake/firedrake/function.py", line 349, in sub
    return self._components[i]
  File "/usr/lib/python3.10/functools.py", line 981, in __get__
    val = self.func(instance)
  File "/home/thomas/firedrake/src/firedrake/firedrake/function.py", line 333, in _components
    return tuple(type(self)(self.function_space().sub(i), self.topological.sub(i))
  File "/home/thomas/firedrake/src/firedrake/firedrake/function.py", line 333, in <genexpr>
    return tuple(type(self)(self.function_space().sub(i), self.topological.sub(i))
  File "petsc4py/PETSc/Log.pyx", line 188, in petsc4py.PETSc.Log.EventDecorator.decorator.wrapped_func
  File "petsc4py/PETSc/Log.pyx", line 189, in petsc4py.PETSc.Log.EventDecorator.decorator.wrapped_func
  File "/home/thomas/firedrake/src/firedrake/firedrake/functionspaceimpl.py", line 187, in sub
    raise IndexError("Invalid component %d, not in [0, %d)" % (i, bound))
IndexError: Invalid component 1, not in [0, 1)

This PR fixes that by simply not calling the remove_initial_w routine. Instead we enforce the no-normal flow boundary conditions through the initial projection of the wind.

The final state demonstrates that this still gives the correct initial condition:

[jshipton]

Great - thanks @tommbendall !