Extended Support for Typing in the Symbolic Toolbox

Closes #115 (closed)

DynamicType for Fields

• Update Field to permit DynamicType as its element type
• Update field factory methods to use DynamicType.NUMERIC_TYPE by default
• Update backend and JIT to deal with this change

Refactoring of Type Casts

• Deprecate CastFunc
• Introduce TypeCast SymPy function as a replacement; use the correct SymPy implementation idioms to derive assumptions and handle the special case of booleans
• Introduce shortcut ps.tcast

Extend Documentation

• Add a user guide on working with data types, concerning
  • Default types and automatic typing
  • Explicit and dynamic typing
  • Type casts
  • The type inference algorithm
• Update docs on field creation and typing