Imported AssnsCrosser algorithm from icarusalg [1/3]

[PetrilloAtWork]

AssnsCrosser is an utility in icarusalg helping to hop across chains of associations.
While it's not a silver bullet and it works on some assumptions, it's usually easier than trying to sort out the several association levels. Documentation is inline and currently in ICARUS Doxygen rendering (in icarus namespace).

Given the extensive need of association hopping, I am proposing here to move it into sbnalg, where it can be used by almost the whole art-aware SBN code base.
The header path is changed accordingly, like the namespace that is moved from icarus::ns::util to sbn::ns::util¹. Because of this, this is a breaking change, which is easy to fix.

Dependent pull requests

Connected pull requests are in:

• Imported AssnsCrosser algorithm from icarusalg [2/3] icarusalg#94 (where the equivalent code is removed)
• Imported AssnsCrosser algorithm from icarusalg [3/3] icaruscode#825 (fixes for the breaking change)
  I do not know of any other place where this utility is used (and not much code is aware of icarusalg beside icaruscode).

Reviewers

• @SFBayLaser as the maintainer of ICARUS software repositories;
• @absolution1 as the maintainer of SBND software repositories.

... and that's it because I don't know who else.
This is a technical PR, that only involves details like old and new names and places.
Of course, people are also welcome to contribute comments on the merit of the algorithm, but I expect that this would not have specific consequences for this PR.

The unit test was moved and still passes.
Note that in my (unit) test one of the tests fails due to broken FHiCL configurations (related to ICARUS overlay) which are not related to this PR. This PR did not touch any FHiCL file and is not supposed to break any of those.

[1] ns stands for "namespace" and was meant to remove the ambiguity of icarus::util namespace with util when working in icarus namespace. Whether this trick actually helps is questionable.