Update of the descriptcheck.py and contributing.md to clarify the usage of Descriptor syntax and arrow usage
As a result of unwanted bugs/features due to incorrect arrows in the tightCut.Decay descriptor, I added two things:
- A section in CONTRIBUTING.md to clarify the correct usage of DecayDescriptor syntax in DECfiles
- The decparser for checking new DECfiles now allows the usage of all kind of arrows (-->, ==>, -x>, etc.). All allowed arrows are changed into a thin arrow before comparing the built-descriptor and the descriptor in line 2.
I hope this helps to prevent further errors in the DECfiles and makes the parsing easier.