Fix to factorial.discrete so that it doesn't require custom init_prepare

[SamDuffield]

The current code for factorial.discrete is insufficient as it requires overwriting the init_prepare in discrete.filter. This isn't ideal as factorial code is supposed to work with the cuthbert filters without modification.

I'm wondering about a more holistic fix. Maybe we can replace Factorializer with FactorialFilter which still has the extract, join functions etc but also has init_prepare, filter_prepare, filter_combine which are just inherited from cuthbert or wrapped in the case of init_prepare. Although this may create too much code clutter. Will think about it some more

[Sahel13]

I'm wondering about a more holistic fix. Maybe we can replace Factorializer with FactorialFilter which still has the extract, join functions etc but also has init_prepare, filter_prepare, filter_combine which are just inherited from cuthbert or wrapped in the case of init_prepare. Although this may create too much code clutter. Will think about it some more

This is related to #245

[SamDuffield]

Fixed with minor modification to cuthbert.discrete.filter doesn't change anything for the user there though