[feature #778][gaia] Apsley and Stansby formulation for slope effect and sliding [R. Kopmann]
- new option for slope effect, available for all bedload threshold formulas
- new option for sediment slide
- keyword SEDIMENT SLIDE: from logical to integer
- new test case for sliding
- update user doc
- modification of figures for yen example and corresponding validation doc
- closes issue #778 (closed)