Add dynamic array type
Adds a dynamic type for array symbols (with known shape) such that the dtype can be inserted later:
TypedSymbol(name, DynamicNumericArrayType(shape))
The typifier (currently) makes no attempt at inferring the shape of untyped array symbols.
Therefore, their shape must be specified explicitly by specifying a type.
Prior to this MR, this had to be a PsArrayType which requires a concrete scalar type.
I wanted to solve this issue by extending the DaynamicType enum with a third variant, which takes the shape as a member (using a similar solution as here).
Unfortunately, that is incompatible with SymPy because nested types can not be pickled