Change symbol typification: Infer unknown LHS data types from RHS
This MR extends the typifier to infer the data type of untyped LHS symbols from an Assignment's RHS.
This feature is required when pystencils extracts subexpressions of non-default type, such as long boolean expressions from piecewise definitions.
Also, a few additional test cases for the typifier are added.
Edited by Frederik Hennig