Refactor DEFAULTS & fix bugs concerning data types of spatial counter symbols

• Remove genericity from pystencils.DEFAULTS, use TypedSymbols only
• Use DynamicType.INDEX_TYPE in counter symbols
• Fix a bug where the SparseIterationSpace declared the spatial counters const, leading to a type conflict with the non-const counters in DEFAULTS
• Add test cases for usage of spatial counters in dense and sparse kernels
• On the side: Add py.typed to facilitate type checking in downstream projects (why was this missed for so long?)