SPZ pollutes `include` root installation path

[hobu]

SPZ currently installs the following includes:

-- Installing: /mypath/include/load-spz.h
-- Installing: /mypath/include/splat-c-types.h
-- Installing: /mypath/include/splat-types.h

It would be cleaner to stick these in an spz prefix path like so:

-- Installing: /mypath/include/spz/load-spz.h
-- Installing: /mypath/include/spz/splat-c-types.h
-- Installing: /mypath/include/spz/splat-types.h

A couple of good reasons for this practice:

• /mypath/include is likely to be marked a system path that has specific warnings shut off for some compilations
• Packaging systems might start sticking your include files there anyway, along with maintaining patches to do so
• It is much easier to read where the include file is coming from #include <spz/load-spz.h>