test: Update regtest reference file to remove nan

Author: giacomofiorin

namd/tests/interface/003_reinitatoms/AutoDiff/test.colvars.traj

@@ -43,7 +43,7 @@
           39    6.88300617631749e+00 -3.72442767426713e+01 -8.83006176317490e-01 
           40    6.88218411453200e+00 -2.66371930519242e+01 -8.82184114532001e-01 
 # step         rgyr                  ft_rgyr               fa_rgyr               
-          40    6.79302548471894e+00                  -nan -7.93025484718935e-01 
+          40    6.79302548471894e+00  0.00000000000000e+00 -7.93025484718935e-01 
           41    6.79123478804224e+00  1.47884739101504e+01 -7.91234788042243e-01 
           42    6.78960135015216e+00  6.31388772773012e+01 -7.89601350152160e-01 
           43    6.78822556441034e+00  1.06473629117400e+02 -7.88225564410344e-01