ENH: Multidimensional image-sequence

`ImageSequence3D` class derived from `ImageSequence`.
Imports t, z, c indexed .tif files. Output is indexed with t index and can be 2/3/4 dimensional.
Would `identifiers` or `tcz_identifiers` be more legible? I don't feel strongly, just offering possibilities. @caspervdw, we can go with your personal preference here.