Allow users to directly access Readers

Currently when a `Reader` is initialised, it is initialised with a filename - this means each `Reader` object can only read one file. Users are currently not expected to use `Reader` instances by themselves, but rather they are used by the `Control` object that the user creates. `Control` also currently creates a new `Reader` instance for every single data file as well, but users may want to access `Reader`s directly in future (to work with data without refining it). However, if this is not required by users then we would be adding unnecessary complexity by providing this (hence why it is in the icebox)

Proposed solution:
split each `Reader` class into a (e.g.) `MantidSQwReader` class with an `open()` method that creates a (e.g.) `MantidSQwFile` object, then the context manager is entered via `with MantidSQwReader.open(filename):` similar to how the regular Python `open()` function for files works