Feature request: streamline from_impedance method

[cmichelenstrofer]

We often start with an impedance as a numpy array and need to convert it to xarray before passing to WEC.from_impedance(). The process is repetitive and takes up many lines in notebooks... Maybe we can put a method either in core or utilities that handles this? here is the relevant code example:

# convert to xarray
freq_attr = {'long_name': 'Wave frequency', 'units': 'rad/s'}
dir_attr = {'long_name': 'Wave direction', 'units': 'rad'}
dof_attr = {'long_name': 'Degree of freedom'}
dof_names = ["Pitch",]
ndof = len(dof_names)
directions = np.atleast_1d(0.0)
ndirections = len(directions)

dims_exc = ('omega', 'wave_direction', 'influenced_dof')
coords_exc = [
    (dims_exc[0], np.squeeze(omega), freq_attr),
    (dims_exc[1], directions, dir_attr),
    (dims_exc[2], dof_names, dof_attr),
]
attrs_exc = {'units': 'N/m', 'long_name': 'Excitation Coefficient'}
exc_coeff = np.expand_dims(np.squeeze(exc_coeff), axis = [1,2])
exc_coeff = xr.DataArray(exc_coeff, dims=dims_exc, coords=coords_exc,
                        attrs=attrs_exc, name='excitation coefficient')


dims_imp = ('radiating_dof', 'influenced_dof', 'omega')
coords_imp = [
    (dims_imp[0], dof_names, dof_attr),
    (dims_imp[1], dof_names, dof_attr),
    (dims_imp[2], np.squeeze(omega), freq_attr),
]
attrs_imp = {'units': 'Ns/m', 'long_name': 'Intrinsic Impedance'}

Zi = np.expand_dims(np.squeeze(Zi), axis=[0,1])
Zi = xr.DataArray(Zi, dims=dims_imp, coords=coords_imp, attrs=attrs_imp, name='Intrinsic impedance')

I think we could make this general (any number of DOFs, any number of wave directions).