Add a length assertion on FilterSets

[thclark]

Feature request

Use Case

I want to be sure that after applying filters, I want to validate that I have a certain number of files.

For example, if I'm expecting a dataset to contain separate files for U, V, W components of a velocity field, I expect to get three files. Maybe I'd do:

filtered = my_dataset.files.filter(name__ends_with=".asc").filter(any_label_contains="component").order_by("tags__component")

But I'll always need to know there'll be three files.

Current state

I'll do:

assert(len(filtered) == 3)

However, I think it'd be neater to have an assertion built in, like:

my_dataset.files.filter(name__ends_with=".asc").filter(any_label_contains="component").order_by("tags__component").assertNumberOfFiles(3)

That way we could ultimately handle the exception and raise it as the same kind of validation error as we get e.g. if the input_values don't validate, so to an external user of the child, validation errors appear unified.

[thclark]

We should probably discuss this as part of a wider subject - unifying validation of file specifications.

[cortadocodes]

Do you still want this @thclark?