Shader-program abstraction

[wpumacay]

Description

These are a few missing pieces in the current implementation of the shader and program objects:

• Replace the usage of std::unique_ptr in favor of std::shared_ptr. This is required as we'll be sharing ownership between the user and containers that have accessors in Python land.
• Add missing manual documentation (.rst) and autogenerated from docstrings (Sphinx).

Tasks

• Use shared_ptr instead of unique_ptr
• Add examples of usage in the docs (.rst)
• Add API reference from generated docs using Sphinx.