message: Fixed gss namespace
diff: - Gudhi::witness_complex::Strong_witness_complex
  - Gudhi::witness_complex::Euclidean_strong_witness_complex
  - The construction of both of them follow the same scheme:
  + The construction of the Euclidean versions of complexes follow the same scheme:
  1. Construct a search tree on landmarks (for that Gudhi::spatial_searching::Kd_tree_search is used internally).
  - 2. Construct lists of nearest landmarks for each witness (special internal structure Gudhi::witness_complex::Active_witness is used internally).
  + 2. Construct lists of nearest landmarks for each witness (special structure Gudhi::witness_complex::Active_witness is used internally).
  3. Construct the witness complex for nearest landmark lists.
  + In the non-Euclidean classes, the lists of nearest landmarks are supposed to be given as input.
  +
  The constructors take on the step 1, while the function 'create_complex' executes the steps 2 and 3.
  \section witnessexample1 Example 1: Constructing weak relaxed witness complex from an off file
  