previously install path was at includedir/fourdst/fourdst/constants, this changes the install path to includedir/fourdst
BREAKING CHANGE: This change may break builds which did not previously use -I/usr/local/fourdst/ (i.e. if previous code included #include "fourdst/fourdst/constants/const.h" it will no longer function). Rather, code should use -I/usr/local/include (or whatever include dir was set by the user at build time) and then #include "fourdst/constants/const.h")
