Add support for limited loading as Array type

[tlnagy]

For packages like ImageIO where we are emitting Arrays, there should be an option for TiffImages to load the file in a simplified and compatible way, i.e. only the largest slice in a pyramidal image, etc. This would insulate users of those packages from TiffImages types and complexity.

This would be more efficient/more general than doing the conversion post-hoc.

Originally posted by @tlnagy in JuliaIO/ImageIO.jl#25 (comment)

[johnnychen94]

A better option is to perhaps separate the pure pyramidal array definition into a lighter package so that ImageIO can depend on it.

This can be quite similar to representing an indexed image as IndirectArray. See also JuliaIO/ImageIO.jl#48