Update get_crosslink_committees_at_slot()

[benjaminion]

Description

As an implemeter, I want get_crosslink_committees_at_slot() to match the v0.4 spec so that Artemis performs as other clients expect.

Acceptance Criteria

• get_crosslink_committees_at_slot() matches the 0.4 specification

Additional Information

Just a minor re-ordering of the controlling if/else statement, I think (branches 1 & 2 need to be reordered), but the function would benefit from a thorough check against the spec.

See ethereum/consensus-specs#672

[benjaminion]

Closed by #440