Changed 'material_definition.py' to 'definition.py'. FILESYSTEM.

This commit changes all material_definition.py modules to just definition.py.
These material definition.py modules are found in hand-made material packages.
(Note that you can have a material package with no definition module: managed
material packages don't rely on definition modules.)

FILESYSTEM: you should change any material_definition.py modules in your score
packages to definition.py.

Here's some code to effect the renames:

    for directory_path, subdirectory_names, file_names in os.walk('.'):
        for file_name in file_names:
            if file_name == 'material_definition.py':
                file_path = os.path.join(directory_path, file_name)
                new_file_path = os.path.join(directory_path, 'definition.py')
                command = 'git mv {} {}'
                command = command.format(file_path, new_file_path)
                os.system(command)