Raise an exception when using `CziFile.size` property if the file has variable per scene dimensions

[evamaxfield]

Currently if a CZI file has variable dimension sizes per scene, the CziFile.size property returns a tuple with length of the number of dimensions but filled with -1. (Ex. (-1, -1, -1, -1, -1, -1)) This is okay but it isn't explicit in why those values are what they are.

I would raise an error that says something like: "File has variable per scene dimension sizes. Use dims_shape instead"