mrregister: fix lmax check for nonlinear multi contrast registration

[maxpietsch]

Fixes the error reported in the community. Also added more verbose error messages to population_template

[maxpietsch]

Pylint seems to be broken for python2:

logging to "pylint.log"
Using pylint for python2 (from PYTHON environment variable)
/usr/bin/python2: No module named pylint
testing "build"... ERROR

Also on repeated CI run. I think CI uses pylint 2.4.4-2 but >2.0 allegedly is not supported for python 2.7.

[maxpietsch]

Couldn't get pylint to work for python2 so removed it from the checks assuming we're going to drop python2 support anyway (#2047).

[jdtournier]

I guess not having pylint for Python2 is not a big deal, and certainly not worth holding this up for, as we're phasing out Python2 support anyway... 👍