-export_grad_fsl: Clamp some b-values to zero

Where the gradient direction vector is zero, but the b-value is non-zero, when FSL's "eddy" attempts to rotate the directions of b>0 volumes according to subject rotation, this results in corrupted gradient directions.
This change prevents such data from reaching "eddy" within dwifslpreproc, by detecting volumes where the gradient vector is zero and the b-value is non-zero but is classified by MRtrix3 as non-zero based on BZeroThreshold and forcing the value within the bvals file to be zero.
Relates to #2577.

core/dwi/gradient.cpp

@@ -178,6 +178,8 @@ namespace MR
     {
       const auto grad = parse_DW_scheme (header);
 
+      size_t bval_zeroed_count = 0;
+
       // rotate vectors from scanner space to image space
       Eigen::MatrixXd G = grad.leftCols<3>() * header.transform().rotation();
 
@@ -191,7 +193,12 @@ namespace MR
         bvecs(0,n) = header.stride(order[0]) > 0 ? G(n,order[0]) : -G(n,order[0]);
         bvecs(1,n) = header.stride(order[1]) > 0 ? G(n,order[1]) : -G(n,order[1]);
         bvecs(2,n) = header.stride(order[2]) > 0 ? G(n,order[2]) : -G(n,order[2]);
-        bvals(0,n) = grad(n,3);
+        if (!G.row(n).squaredNorm() && grad(n, 3) <= MR::DWI::bzero_threshold()) {
+          ++bval_zeroed_count;
+          bvals(0,n) = 0.0;
+        } else {
+          bvals(0,n) = grad(n,3);
+        }
       }
 
       // bvecs format actually assumes a LHS coordinate system even if image is
@@ -200,6 +207,11 @@ namespace MR
       if (adjusted_transform.linear().determinant() > 0.0)
         bvecs.row(0) = -bvecs.row(0);
 
+      if (bval_zeroed_count) {
+        WARN("For image \"" + header.name() + "\", " + str(bval_zeroed_count) + " volumes had zero gradient direction vector, but 0.0 < b-value <= BZeroThreshold; "
+             "these are clamped to zero in bvals file \"" + bvals_path + "\" for compatibility with external software");
+      }
+
       save_matrix (bvecs, bvecs_path, KeyValues(), false);
       save_matrix (bvals, bvals_path, KeyValues(), false);
     }