Fpsim2

fixed regression issues with tversky and substructure searches

added distance matrix feature

- use libpopcnt

- some static typing

- some cleanup

- update ubuntu distro in travis, compile with gcc-5 to allow c++17

- do not use processes in a single core on disk search

- update setup.py

- clean gil release

- added tversky search with a b params

* fix 36