Fixes to Mixture Parameters

If F_ij is zero, put in a dummy departure function
If pair is invalid, throw error rather than crash

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Removed printing of indices for maxima of phase envelope

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Added first cut at debian

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Merge branch 'master' of https://github.com/coolprop/coolprop