FRQ: allow user to supply part of a trajectory as trajectory

[kay-ro]

Proposed new feature or change:

Rather than having to supply a full trajectory, it would be useful if the user also can only supply parts of it.

traj[:100:] or traj[::2] currently do not work because the required type in most functions is FieldTrajectory | ParticleTrajectory rather than list | iterable of Frame. If a subset of a trajectory is still returned as a trajectory, traj[i] should still return the ith frame and not a trajectory of length 1 (for compatibility!).

Implementation principle:

class A:
    def __init__(self, liste, sl=None):
        self.__liste=liste
        self.__allitems=np.arange(len(liste))
        if sl is None:
            self.__sl=slice(0,len(liste),1)
        else:
            self.__sl=sl
    def __getitem__(self, item):
        if isinstance(item, int):
            item = self.__allitems[self.__sl][item]
            return self.__liste[item]
        elif isinstance(item, slice):
            # -> convert slice to list
            # create class A from sl=sl-list[item]
            # (np.arange(100)[slice(0,10,1)])[slice(0,6,2)]
            return A(self.__liste, sl=item)
        elif isinstance(item, list):
            # create class A from sl=sl[item]
            return A(self.__liste, sl=item)
    def __iter__(self):
        for i in range(len(self.__liste[self.__sl])):
            yield self[i]
    def __len__(self):
        return len(self.__liste[self.__sl])

a = A(np.arange(0,100,1))

print(len(a), type(a), a[13])
b = a[[1,6,3]]
print(len(b), type(b), b[1])
c = b[::2]
print(len(c), type(c), c[13])

100 <class '__main__.A'> 13
3 <class '__main__.A'> 6
50 <class '__main__.A'> 26