[xMapping] rm warning about potential non initialized variable usage
In both files some variable were potentially not initialized and nevertheless used for some computation. After checking with N.C. for xLagrangeMapping.cc the case were knots.size() is null is impossible => assert. Then a simple init with first point remove the warning and set without ambiguity x1,y1,z1. For xMapping.cc the switch was not defaulted and thus DetJac may remain unset. Setting it to 1 and put a default case in switch solve the prb.