[MGT] Fix search length algo in MeshNodeSearcher.

In case of aquidistant meshes the expression sum_of_sqr - (sum*sum)/edge_cnt can
get negative due to numerical calculation. The root of a negative value is not
computable in real numbers.
[MGT] Add assertion in MeshNodesAlongPolyline algo

Epsilon must be a positive value.
[MGT] Typos, empty doxygen fields, include paths.
[MGT] Variables rename, static_cast instead of dynamic.
[MGT] Replace iterative search with explicit solution.