Refactor Field Indexing Symbols

This MR mainly refactors and clarifies the types and semantics of field shape/stride/pointer symbols.

• Remove field shape, stride, and pointer symbols from frontend
• Extend TypedSymbol to accept DynamicTypes
• Make Field use DynamicType for its indexing symbols so that the indexing type is determined during translation
• Consolidate index symbol names in DEFAULTS
• Allow passing arbitrary names for array indexing symbols
• Fix index type checking in KernelCreationContext.add_field
• Revert to pystencils 1.x naming convention for field pointers (due to compatibility issues in waLBerla)