Support slicing properties as properties (aka. support obj[i:j].shape).

[anntzer]

Currently, after sliced = pims.open(...)[i:j], the sliced object doesn't have a .shape attribute (#38), even though it does have a len() and a .frame_shape (the latter being propagated via propagated_attrs) and even though FramesSequence.shape is actually a property (return (len(self), *self.frame_shape)).

This PR adds the ability to mark properties as being propagated, not as values, but rather as properties in the sliced object too (the decorator name, @sliceable_property, can be bikeshedded). This is done by having __getitem__ not return a Slicerator, but a subclass of Slicerator with the relevant properties.

While we're at it, "standard" propagated attributes are also implemented using properties now (which directly call getattr on the ancestor, not going through any fget); this is a slight API change in that later changes to the attribute on the ancestor will now be reflected on the sliced object, but the keeping the implementation consistent with sliceable properties seems nice and having these attributes tab-completable (rather than being hidden behind __getattr__ is good for usability). In any case this could be changed back if the API change is problematic.

As a result, with this PR, pims/base_frames.py just needs a small extra patch:

diff --git i/pims/base_frames.py w/pims/base_frames.py
index d91df98..122b12d 100644
--- i/pims/base_frames.py
+++ w/pims/base_frames.py
@@ -1,6 +1,6 @@
 import numpy as np
 import itertools
-from slicerator import Slicerator, propagate_attr, index_attr
+from slicerator import Slicerator, propagate_attr, index_attr, sliceable_property
 from .frame import Frame
 from abc import ABC, abstractmethod, abstractproperty
 from warnings import warn
@@ -92,7 +92,7 @@ class FramesSequence(FramesStream):
     Must be finite length.

     """
-    propagate_attrs = ['frame_shape', 'pixel_type']
+    propagate_attrs = ['frame_shape', 'pixel_type', 'shape']

     def __getitem__(self, key):
         """__getitem__ is handled by Slicerator. In all pims readers, the data
@@ -102,7 +102,7 @@ class FramesSequence(FramesStream):
     def __iter__(self):
         return iter(self[:])

-    @property
+    @sliceable_property
     def shape(self):
         return (len(self), *self.frame_shape)

to support sliced = pims.open(...)[i:j]; sliced.shape.

I can add tests and make the corresponding change in pims if we agree on the design here.

[anntzer]

@nkeim @tacaswell Any interest in pushing this PR (or something similar) forward? As noted above this is really motivated by adding support for .shape on sliced pims objects.