Return integer instead of string for the CDS xmatch

[JulienPeloton]

The crossmatch module returns the type of object as a string, based on the classification at http://simbad.u-strasbg.fr/simbad/sim-display?data=otypes. But then, we use this classification to link alerts of the same kind (in grafink for example), which can be slow. Comparing integers would be faster than strings.

Action item: return the integer type instead of the string one, and build a mapping to perform the conversion if necessary. The mapping can be accessed at http://simbad.u-strasbg.fr/simbad/sim-tap (with SELECT * FROM otypedef).