Feature/yaml instrument

I added the support for yaml for all the instruments. The code is now fully free of boost::property_tree. Could yo please have a look at the yaml files and code changes to check that everything is OK from your side and that they match your expectations ? A few points may be discussed at this stage:

• is offset still in used after all you recent refactoring work ? If no I can skip it from the YAML file
• the use of units is not systematic. For instance, the position key has no unit while the width and height are defined in most of the xml/yaml files with mm. On the other hand the key wavelength has no unit in the xml/yaml. Indeed, it is implicitly considered to be in angstrom. This is not consistent. Should we get rid of units throughout all the code by using SI systematically ?