Merge branch 'normalize_descent' into 'master'

normalize descent direction for SteepestDescent and VL_BFGS (but not for RelaxedNewton!)

See merge request !188
5 jobs for master in 18 minutes and 43 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #251965
test_min

00:03:47

passed #251966
test_mpi

00:05:07

passed #251967
test_mpi_fftw

00:05:50

passed #251968
test_mpi_fftw_hdf5

00:12:14

66.0%
 
  Release
passed #251969
release_image_master

00:06:28