Synchronise leaf movie frame and cell export data timestep references.

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user: Michael Guravage <michael.guravage@cwi.nl>
branch 'default'
changed doc/v1.html
changed doc/v1.pdf
changed doc/v1.rst
changed src/ChangeLog
changed src/VirtualLeaf.cpp
changed src/canvas.cpp
changed src/canvas.h
changed src/mainbase.h
changed src/mesh.h