Added function 'testplot' in module 'useful'.

Signed-off-by: Eduardo Tenorio <embatbr@gmail.com>
Remodeling function 'frame_signal' in module 'sigproc' (and where it is referenced: module 'features') and creating a module 'sigproc_test' to extract (not finished yet) the tests from section 'if-main'.

Signed-off-by: Eduardo Tenorio <embatbr@gmail.com>
Finishing module 'sigproc_test', removing test code from module 'sigproc' and refactoring function 'testplot' in module 'useful'.

Signed-off-by: Eduardo Tenorio <embatbr@gmail.com>