OPEN ISSUES:
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Issue: Segments is pmesh used to separate regions 
(e.g. for different resolution) will appear as an *internal* 
boundary in the resulting domain even though it isn't intended to have a boundary condition applied.
Background: Genuine internal boundaries are created the same way as 
external boundaries through the 'boundary' dictionary mapping triagle_id 
and edge number to a tag.
When a mesh is created, this dictionary will override the neighbour structure 
and thereby enforce the internal boundary status.
However, once a segment belongs to a boundary (internal or external) it must be bound to a boundary condition object in order to supply values for the 
flux computations.
An older version of pyvolution allowed None as a boundary object which 
probably meant that a zero dirichlet condition was imposed. 
Not sure about this, though.
Importance: High
Suggested Action: Have pmesh ignore segments that aren't being tagged
 and that way they are only used to to define regions 
 (e.g. for variable resolution). 
Status: Unresolved