WebSubmit: new option `layer' for stamping tool
- Added 'layer' option to WebSubmit stamping tool in order to choose the layer to stamp (background | foreground). Closes #58
- Fixed usage of the 'latex_template_vars' parameter of the DEMOTHE submission. Clarified usage of this parameter in the documentation of functions "Stamp_Replace_Single_File_Approval.py" and "Stamp_Uploaded_Files.py"
- Clarified usage of parameter 'output-file' of the stamping tool (CLI + APIs).
- Added stamping tool tests.