tutorial/ -> tutorials/

initializing the api now sets General.Terminal=1 (to print messages by default) and General.AbortOnError=2 (to throw an exception as soon as an error happens) - cf. #1043

changed API for creation of simple entities: the tag is now optional, and provided at the end (with default value = -1)

- all functions now return a vector<int>
- first entry is the return code
- any integer output value is appended

This makes it as natural to use when specifying tags explicitly or when letting Gmsh return new ones