Added finalized tutorial code. Added new header files that provide simplified constructs for summing up concentrations in walls. Also added a new function for resetting all walls of a cell.
New menu items in canvas for resetting chemical values only in cells or only in walls.

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user: Roeland Merks <roeland.merks@cwi.nl>
branch 'default'
added src/TutorialCode/Tutorial1A/mymodel.cpp
added src/TutorialCode/Tutorial1A/mymodel.h
added src/TutorialCode/Tutorial1A/mymodel.pro
added src/TutorialCode/Tutorial1B/mymodel.cpp
added src/TutorialCode/Tutorial1B/mymodel.h
added src/TutorialCode/Tutorial1B/mymodel.pro
added src/TutorialCode/Tutorial1C/mymodel.cpp
added src/TutorialCode/Tutorial1C/mymodel.h
added src/TutorialCode/Tutorial1C/mymodel.pro
added src/TutorialCode/Tutorial1D/mymodel.cpp
added src/TutorialCode/Tutorial1D/mymodel.h
added src/TutorialCode/Tutorial1D/mymodel.pro
added src/TutorialCode/Tutorial3/tutorial3_init.xml
added src/TutorialCode/Tutorial4/auxin_growth_init.xml
added src/TutorialCode/Tutorial4/mymodel
added src/TutorialCode/Tutorial4/tutorial4_init.xml
added src/TutorialCode/Tutorial5/mymodel
added src/TutorialCode/Tutorial5/tutorial5_init.xml
added src/flux_function.h
added src/vleafmodel.h
changed src/Makefile
changed src/TutorialCode/Tutorial2/mymodel.cpp
changed src/TutorialCode/Tutorial3/mymodel.cpp
changed src/TutorialCode/Tutorial4/mymodel.cpp
changed src/TutorialCode/Tutorial4/mymodel.h
changed src/TutorialCode/Tutorial5/mymodel.cpp
changed src/TutorialCode/Tutorial5/mymodel.h
changed src/VirtualLeaf.pro
changed src/cellbase.cpp
changed src/cellbase.h