Remove LinearLocator, LogLocator from Visualization

[git-abhishek]

PR Summary

Cleanup by removing LinearLocator, LogLocator and FieldTransform.

PR Checklist

• Code passes flake8 checker

Adding Reviewers

@ngoldbaum @Xarthisius @colinmarc

[git-abhishek]

@ngoldbaum Does the LogLocator implementation complete and correct? One thing is that it does not allow to pass in user defined numticks.

[matthewturk]

I think this is a good change, but I'd go even further to get rid of the FieldTransform objects altogether and just use strings.

[ngoldbaum]

And in places where you need to use the funciton (where is that happening btw?) you could either use a lambda or just define a function outside of the FieldTransform class and use the functions where they're needed.

[git-abhishek]

Its happening here.

self._field_transform[field] = symlog_transform
self._field_transform[field].func = linthresh

How should this be handled, if we want self._field_transfrom[field] to be just string?
My earlier commit failed due to this error:

ERROR: yt.visualization.tests.test_plotwindow.test_symlog_colorbar

Traceback (most recent call last):
File "/home/travis/build/yt-project/yt/venv/lib/python3.6/site-packages/nose/case.py", line 198, in runTest
self.test(*self.arg)
File "/home/travis/build/yt-project/yt/yt/visualization/tests/test_plotwindow.py", line 551, in test_symlog_colorbar
plot.set_log(field, True, linthresh=0.1)
File "/home/travis/build/yt-project/yt/yt/visualization/plot_container.py", line 76, in newfunc
rv = f(*args, **kwargs)
File "/home/travis/build/yt-project/yt/yt/visualization/plot_container.py", line 219, in set_log
self._field_transform[field].func = linthresh
AttributeError: 'str' object has no attribute 'func'

[ngoldbaum]

Ah I see, we do actually need the function, it gets passed to matplotlib here:

https://github.com/yt-project/yt/blob/master/yt/visualization/plot_window.py#L842-L847

So I guess we really do need both a name and a function, so it makes sense to keep the FieldTransform object. I guess we could use a tuple or something like that, but I think that would end up less clear.

I think this is fine as-is.

[matthewturk]

Looks good!