Reverted change to gyro-averaging flags

Default flags have boundary region width of 1. Changing this
broke the test case, so have reverted.
Fixed BoutMesh::GlobalY for double-null plasmas

Doesn't affect most cases, only where equilibrium is double null.
Now provides a continuous y in [0,2pi] for constructing initial conditions.

In the lower PF region y jumps from 0 to 2pi across the branch cut.