Fix incorrectly reduced variables in nimhd test

[conradtchan]

Type of PR:
Bug fix

Description:
Required for #217

In a93bc90, as part of implementing MPI support for nimhd tests, the L2 error variables were reduced across tasks.

There is a typo resulting in the v_x and B_y error being overwritten by the density error. This produces a test failure due to the tolerance not being met.

In df72df1, NICIL 2.1 was added to phantom, where the tolerance for B_y was changed to tolb=3.1d-2. This was change was made possibly because the apparent improved accuracy was wrongly attributed to improvements to the code, rather than because the the density error was overwriting the B_y error.

tolb has been reverted to the original value of 1.1d-1.

This bug is not yet covered by the automated test suite because #217 has not been merged yet, and needs to be tested manually using the commands below.

Testing:

make MPI=yes SETUP=testnimhd OPENMP=no phantomtest
mpirun -n 4 bin/phantomtest nimhd

Did you run the bots? no, not required