add support for slice

Iterator was extended with support for the python slice (getitem) syntax

The ogr documentation states for the functions:
OGR_L_GetExtent
OGR_L_GetExtentEx
OGR_L_GetFeatureCount
OGR_L_GetFeature
that they can modify the read cursor. A check for such situations was added. In case a function call to one of these methods newer than the start of a sequential read is detected, a runtime warning is given.

Not so nice is, that getitem is implemented on iterator and not collection. However, this allows to reuse it for filter with a boundingbox.