Add missing connectivities in IconGrid

[tehrengruber]

The IconGrid is missing some connectivities (that are present in the simple_grid). For example 7696d1a introduced the offset provider for C2CECEC, but we do not construct the respective connectivity.

[philip-paul-mueller]

@tehrengruber Here is my implementation for E2C2E/E2C2EO it might be of use to you.

def compute_e2c2eO(
        c2e, e2c,
        with_origin = True):
    """Generate the `E2C2E0` map based on the `C2E` and `E2C`.

    The operation is relatively costly, since it has to be done in Python.
    By default the origin edge is included, however, by setting `with_origin` to `False` the function will generate `E2C2E`.
    """

    # You are not supposed to understand this
    dummy_c2e = np.append(c2e, -np.ones_like(c2e), axis=1)
    T  = dummy_c2e[e2c, :]
    Ts = T.shape
    S  = T.reshape(Ts[0], Ts[1] * Ts[2])
    
    # Now we get all the unique indexes
    u = []
    highestCount = -1
    for i in range(S.shape[0]):
        s = S[i]
        s = s[s >= 0]       # Remove the invalid indexes.
        if(with_origin):    # Add the origin edge ID if requested.
            s = list(s) + [i]
        else:               #  or if not, ensure that the ID is not present.
            s = s[s != i]
        #
        u.append(np.unique(s))  # Add the unique indexes.
        highestCount = max(highestCount, u[-1].size)
    #

    # Now combine all indexes inside a single array index.
    _e2c2eO = np.full((len(u), highestCount), -1, dtype=c2e.dtype)
    for i, s in enumerate(u):
        _e2c2eO[i, 0:len(s)] = s
    #
    return _e2c2eO
# end def: compute_e2c2eO