Removed Mixtures files due to new structure

Signed-off-by: Ian bell <ian.h.bell@gmail.com>
Renamed phi to alpha for non-dimensional Helmholtz energy terms

Signed-off-by: Ian bell <ian.h.bell@gmail.com>
Fixed docs for HE EOS classes

Signed-off-by: Ian bell <ian.h.bell@gmail.com>
Flattened backends back into CoolProp namespace

Signed-off-by: Ian bell <ian.h.bell@gmail.com>
Changed phi to alpha in abstract state

Signed-off-by: Ian bell <ian.h.bell@gmail.com>