Utilities to merge associate blocks and restrict depth of associate resolution

[mlange05]

Note: This sits on top of PR #387 and requires this to be merged first.

This PR adds a new utility to merge Associate blocks and adds a feature to the existing resolve_associations method to restrict the starting depth of the association resolution. Combining the two allows for a neat look and feel, where all except the first layer of (non-nested) associations get resolved after all non-nested ones have been merged into the outermost associate block. Tests have been added for both features.