Split up and rearrange files

[simonbyrne]

The main HDF5.jl file is rather large, and very difficult to navigate, especially since the API is not yet fully documented. I think this would be easier to develop and maintain if we were to split this up into smaller files. I originally tried this in #833, but it was quite a big change, so instead I decided to tackle it smaller chunks.

The question is what should go where. @mkitti suggested I write it up as an issue so that it can be discussed in one place.

My overall idea is to split it out according to the how the C API is broken up

• Properties (h5p): done in Refactor Properties interface #847
• File functions (h5f): File type and h5open: move file-related functions to new file #970
• Group functions (h5g): Group type and setindex!/getindex overloads: move group and link functions to new file #971
• Datatype (h5t): Datatype and conversions between Julia types and HDF5 datatypes (move datatype functions to new file #975)
• Dataspace (h5s): Dataspace and conversions between Julia indexing and dataspaces: See move dataspace-related functions to new file #961
• Dataset (h5d): Dataset and read/write functionality: see Move code into datasets.jl, readwrite.jl, and types.jl #955.
• Attributes (h5a): See move attributes to new file, add docs #943
• References (h5r): Reference type and dereferencing: see move Reference pieces to new file #968
• Objects (h5o): See move object-related functions to new file #976