Allow ImageFileCollections to take a list

[crawfordsm]

Instead of giving a directory as the entry to ImageCollections, it should also be able to take a list of images so the user can specify exactly what files they want included.

[mirca]

Hi @crawfordsm,

should the files be assumed to be in location or should it be any list of relative (or absolute) paths?

I was thinking in add a parameter which would be the list of files specified by the user wrt to location, is that the intention?

[crawfordsm]

@mirca maybe both? If location is supplied then an absolute path not needed, but if location is None then it would need an absolute path

[crawfordsm]

One part of this has been implement in #403, but different locations still needs to be implemented.

[mwcraig]

What if, instead of switching based on image location, we do the Unix-y thing of using an absolute path if the filename starts with / but otherwise we use pre-pend location?