Changed ExMODEL parameters to ELLIPx_MODEL.

Changed EPSxMODEL parameters to POLARx_MODEL.
Added AMODEL, BMODEL, THETAMODEL for both _IMAGE and _WORLD coordinates.
Added CXXMODEL CYYMODEL, CXYMODEL for both _IMAGE and _WORLD coordinates.
AddedTHETAMODEL_SKY,THETAMODEL_J2000,THETAMODEL_B1950.
Added ELLIPxMODEL_WORLD and POLARxMODEL_WORLD.
Fixed broken PSF-fitting.
Fixed warnings while generating the configure (thanks to S.Pascual).
Pushed version number to 2.9.1.