Merge branch 'master' of https://github.com/DamCB/tyssue

Some ameliorations (DamCB#76)



* Periodic boundary condition implementation

* "visible" face column in 3D sheet view

* better catch Exception with opposite

* fixes a bug in HI transition on an apical surface

Sets integer instead of bool as defaults for masks

changed version number

Merge branch 'master' of github.com:DamCB/tyssue

setting version to 0.2.rc1

Merge branch 'master' of github.com:DamCB/tyssue

Version 0.1