Properly support unlimited dimensions

[meggart]

It is propably not too much work, I personally just don't use unlimited dimensions so I kind of ignored them so far. This needs to change...

[rickedanielson]

The time dimension was traditionally seen to be a good UNLIMITED candidate for files that were expected grow as an evolution was being followed. A previous error ("ERROR: LoadError: BoundsError: attempt to access 1-element Array{UTF8String,1}:") was not related, but seemed to be at first. However, further tests reading a variable whose time dimension is UNLIMITED are passing and perhaps it should be noted (on closing this issue) that unlimited dimensions may not require specific attention...(?)

[meggart]

Yes, for reading unlimited dimensions no action is required. Creating files to and writing unlimited dimensions is possible now with #13 . As I just noticed, this is still undocumented #21 , so another todo...