Missing check on SVD decomposition

[giordano]

If SVD decomposition fails at

Glimpse/src/field.cpp

Line 936 in 3e01fce

svd ( U, s, V, Asmall );

then s is reset and the following instruction s( 0 ) causes the program to terminate with the following error:

error: Mat::operator(): index out of bounds
terminate called after throwing an instance of 'std::logic_error'
  what():  Mat::operator(): index out of bounds

svd function actually returns a bool flag to indicate whether decomposition was successful or not. Glimpse should check the value of the returned flag and deal with the case where the decomposition failed.