Make PBC type enumeration into PbcType enum class

Make PBC type enumeration into PbcType enum class

The type of periodic boundary conditions is defined by untyped
enumeration and passed around as an integer, which is not type
safe. This change makes the enum class that defines the type of
the PBC and can be safely passed as a function argument.

Change-Id: Ia1f709a90472a95ad76af92c41df570ba7c28255