Move PSize.hpp from nix/hdf5/ to nix/ as NDSize.hpp
PSize (Size.hpp): Use generic type instead of hsize_t

At various places hsize_t was hard-coded even though it was
not necessary and it is more correct to use the template
type
PSize: Use (u)int64_t as inner type instead of h(s)size_t

This will make this type generically usable
PSize: move into top-level 'nix' namespace
PSize: Rename to NDSize (same for PSSize)

find . -type f | /usr/bin/grep -E '^.*[h|c]pp$' | xargs perl -i -p -e 's/PSSize/NDSSize/g'
Extract interfaces

- start to separeate the interface from the implementation
- rename folders, namespaces and include guards from "pandora" to "nix"
Merge pull request #31 from gicmo/interfaces_ck

PSize -> NDSize change
Add comment about stability to README.md