Change the abstract method in Filegettermanipulators to "manipulate()" to be consistent with other manipulators

[mjordan]

Fetcher manipulators and metadata manipulators implement an abstract method, ->manipulate(), whereas filegetter manipulators implement a different abstract method, ->getMasterFilePaths(). This anomalous method is implemented in only one subclass, https://github.com/MarcusBarnes/mik/blob/master/src/filegetters/CdmSingleFile.php#L154.

In the interests of a consistent interface to manipulators, we should change the abstract method defined in https://github.com/MarcusBarnes/mik/blob/master/src/filegettermanipulators/Filegettermanipulator.php#L33 to ->manipulate(), and update https://github.com/MarcusBarnes/mik/blob/master/src/filegetters/CdmSingleFile.php#L154 to use the renamed method.

This issue is related to #400 and #406.