reverted back to pow3 as default as mamy runs fail to converge with tanh and correct cov

meloptions.h

@@ -229,7 +229,7 @@ class MelodicOptions {
    approach(string("-a,--approach"),  string("symm"),
 	   string("approach for decomposition, 2D: defl, symm (default), 3D: tica, concat (default)"),
 	   false, requires_argument),
-   nonlinearity(string("--nl"), string("tanh"),
+   nonlinearity(string("--nl"), string("pow3"),
 	   string("\tnonlinearity: gauss, tanh, pow3, pow4"), 
 	   false, requires_argument),
    varnorm(string("--vn,--varnorm"), true,