Speed up by 10-20% by creating full mode array in model.__init__ instead of creating an empty array and appending to it.

If we create a Model and pass an array of modes (_modes), __init__ currently calls this block:

    self.modes = np.empty([0],dtype=modetype)
    """array containing the modes (n, l, freq, inertia)"""

    if (_modes is not None): self.append_modes(_modes)

self.append_modes simply casts _modes as a modetype array and appends it:

    def append_modes(self,modes):
        ...
        self.modes = np.append(self.modes,np.array(modes,dtype=modetype))

It's simpler to do the work of append_modes in __init__, without appending, which is what this merge request does. It also happens to be 10-20% faster, based on some rough profiling, because AIMS creates Model objects when interpolating and resizing the arrays is slower than simply creating them with the correct size in the first place.