Changing the pixel flagging scheme

[anand-avinash]

This PR changes how the pathological pixels are identified and flagged. Earlier, we were using the following two criteria:

1. Comparing the determinant of the blocks of $P^t diag(N)^{-1}P$ for each pixel against zero.
2. Comparing weighted_counts against count threshold (given by SolverType - 1)

We have updated them as follows:

1. Comparing the absolute value of the determinant of the blocks of $P^t diag(N)^{-1}P$ for each pixel against zero.
2. Comparing hit_counts against count threshold (given by SolverType - 1)