Consolidate codegen and JIT modules.

Consolidate all code-generation related front-end APIs into pystencils.codegen module. Consolidate all JIT-related front-end APIs into pystencils.jit.

First part of #116 (closed).

pystencils.codegen

The codegen module consolidates target, config, kernelcreation, backend.properties and backend.kernelfunction. It is home to the code generation drivers, their configuration, invocation APIs, and output objects. The drivers internally invoke the backend; backend APIs are now no longer exposed to the user (apart from some exceptions).

pystencils.jit

The jit module is moved out of backend (where it never really belonged). It shall be the home of all JIT-related APIs and functionality.

docs/source/reference/gpu_kernels.md

@@ -45,7 +45,7 @@ update = ps.Assignment(f.center(), 2 * g.center())
 cfg = ps.CreateKernelConfig(target=ps.Target.CUDA)
 kernel = ps.create_kernel(update, cfg)
 
-ps.show_code(kernel)
+ps.inspect(kernel)
 ```
 
 The `kernel` object returned by the code generator in above snippet is an instance