Improve merge overlap meshes

[felicio93]

Added the capability of smooth transitioning overlapping meshes:

• The user can now either pass a mesh to be used as the transition.
  If no hfun_mesh if passed to merge_overlapping_meshes, it will use the low priority mesh (floodplain, coarse mesh, etc.) as transition.

In addition:

• updated tests

• addressed issue Incorrect handling of str or Path input for collector geom #172

[felicio93]

Passed all tests, closing PR.