Sparse2D headers install path

[EiffL]

@sfarrens I notice that the install path for headers is:

INSTALL(FILES ${inc_lib} DESTINATION include)

I would suggest using a subdirectory, like:

INSTALL(FILES ${inc_lib} DESTINATION include/sparse2d)

This prevents a lot of annoying name confusions in includes by keeping the library clearly isolated.
Then in codes that use that library, one can import things as:

#include <sparse2d/IM_IO.h>

Also, one other thing, the headers for the tools are not installed.

I'm openning a branch to fix these problem and will open a pull request