Mesh Modifications & Load Combinations

[SoundsSerious]

• added an additional selection of load_combo in addition to combo_tags as a union of input choices
• added an angular offset to cylindrical / annular meshes based on the start node
• clean up merge_duplicate_nodes to fit 80 character wide standard by selectiong n1,n2 prior to replacement logic (nodes removed after identification so it should be the same)
• added a KDTree duplicate node identification method
• tests pass!