Move capsule code into py_statements

[ltaylor16]

Replace code generated in wrapp.py with Python code with statements in py_statements. This simplifies the Python while providing the user more control in the YAML file.

Removed py_statement fields:

• allocare_local_var
• cxx_local_var
• declare_capsule, post_call_capsule, fail_capsule
• declare_keep, post_call_keep, fail_keep

Added py_statement fields:

• local - generate local variable names and add to fmt. cxx and capsule will generate the current names to aid migration.
• call
• destructor
• destructor_name

destructor and destructor_name are similar to del_lines and capsule_type which used to exist in the Python wrapper.

Created mixin groups to share code among groups.