Delay cross power spectrum estimator

[jrs65]

Here is a finalised version of the cross spectrum estimator that fits in with Simon's earlier refactoring. To make it work I've had to refactor the base code a bit further to remove some duplication. Notable changes:

• The output container creation has been moved from _process_data into a new _create_output method.
• The delay calculation code is moved into its own method _calculate_delays
• The data/weight manipulation code is moved into _cut_data method.

With these changes above it's reasonably simple to create a new cross spectrum estimator task.

Also in this PR I've merged in some changes I made to work with the absorber project of a mixin to abstract over containers with a "data" dataset and a corresponding weight dataset. That allows the general container transforms to chose the right datasets to use without explicitly specifying them.

[jrs65]

One more thing, for reviewing it may be easier to look through commit by commit...