API adjustments

Several small changes in the API, aiming to make it more usable:

- module info is constant so store it once upon loading the module
- make get_format_info a static method
- add a helper to get module data from module info
- rename get_sample to get_sample_data

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Further API adjustments

Add get_sample method to return the sample object for the given instrument
and sub-instrument numbers, and make get_sample_data a static method that
extracts sample data from the given sample object.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>