Plugin new view element data

See merge request !373

add patch version in API version: it's not strictly necessary (we don't break the API between patch releases), but it makes things simpler.

rationale: with non-breaking API changes (e.g. adding a new function) one had to use gmsh/option/getNumber("General.Version") to check the Gmsh version; using the same numbering for the API and for Gmsh means one can simply examine the API version.

Fixes #997

new optional parameter in geo/addCurveLoop to reorient the curves if necessary + throw exception in api when curve loops are wrong (cf. #993)

if several element layers are prescribed without relative heights, use equidistant heights (instead of discarding the multiple layer constraint altogether, which is less intuitive - see #990)

consistent return type 0 length array

See merge request !372

script e.g. numpy.array((0,),np.float64)+1 is valid but []+1 is invalid

basic global mesh size callback in API

See merge request !371