mrtrix3.image.statistic(): Fix parameter naming

[Lestropie]

app.error() call contained a reference to the function being invoked, rather than the parameter that was passed to the function.

Not hugely consequential given the script is throwing an error at that point anyway; but nevertheless preferable for a more appropriate error message than "TypeError: must be str, not function".

Observed on forum.

[jdtournier]

Same issue here as on #1505, but with added goodness:

## testing "lib/mrtrix3/_5ttgen/gif.py"...
 [ ERROR ]
************* Module mrtrix3._5ttgen.gif
lib/mrtrix3/_5ttgen/gif.py:21:2: W0611: Unused import os (unused-import)
lib/mrtrix3/_5ttgen/gif.py:21:2: W0611: Unused import os (unused-import)
lib/mrtrix3/_5ttgen/gif.py:28:2: W0611: Unused import os (unused-import)
lib/mrtrix3/_5ttgen/gif.py:29:2: W0611: Unused app imported from mrtrix3 (unused-import)
lib/mrtrix3/_5ttgen/gif.py:28:2: W0611: Unused import os (unused-import)

Given that this is all based on master, all parent commits passed testing, and none of the commits since have touched this file, I'm stumped. The only explanation as far as I can tell is that pylint has suddenly decided to become stricter...

In any case, I'll try to fix this one too.

[jdtournier]

I note a stack of errors in the TravisCI logs at the pip3 install pylint stage. Not sure whether the version of pylint that is eventually installed is the right one. I note pylint 2.2 just came out - tests run clean locally on my system if I use the previous 2.1.1 version, but not 2.2 (on top of that, the tests for labelsgmfix cause a frank crash of pylint...). I've just added a line to run_pylint to output the version information so we can see what is running exactly (though I note I'll need to fix that, it's not quite right...).

[jdtournier]

OK, this looks like it works...

@Lestropie: please review these changes and make sure you're happy. There's changes to your GIF algorithm, and to the run_pylint script - both of which are yours. Since this is going to master, I think it's best you give it the thumbs-up (I can't add you as a reviewer since you submitted the PR originally...).

[Lestropie]

The only explanation as far as I can tell is that pylint has suddenly decided to become stricter

On cursory inspection perhaps the new pylint emits unused-import instead of unused-variable, the latter of which was explicitly silenced on the relevant lines (which is acceptable but should only be used where absolutely necessary).

There's changes to your GIF algorithm ...

Not mine.

[thijsdhollander]

Alright, looks like this is fixed then!