Distinguish between SymPy and pystencils Assignement better

This MR clears the usage of SympyAssignement in contrast to the sympy assignement.

In the backend of pystencils only SympyAssignements are used now that inherit from pystencils base Node class

fixes #61 (closed)

Additionally, AddAugumentedAssignement is introduced for convenience

added feature label

requested review from @ob28imeq

assigned to @holzer

added 1 commit

• c55ae064 - Small fiz

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added 1 commit

• cb78a427 - Remove unnecessary if statement

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resolved all threads

approved this merge request

merged

mentioned in commit a861b607