- CharacteristicLength -> MeshSize, geometry Lines->Curves

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

- add "geo" versions of add/RemovePhysicalGroup in anticipation of move of "master" physical group definition in GModel (we will need to keep the built-in kernel defintion as well, since built-in CAD operations track physicals)
- more explicit mentions of CAD kernel in api docs

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