Fix field parameters not working with off axis projections

[jzuhone]

PR Summary

This is an attempt to fix issue #3126 to ensure that field parameters can be used with off-axis projections. This is a work in progress.

I think that I only need to restore the underscore that was removed in this commit:

cde25a8#diff-2d1d24ee7df2e3b516614aad6fa0d7bd09a0268684a1c6b748eee75c1b823002

which is here, in line 57:

yt/yt/visualization/volume_rendering/render_source.py

Lines 52 to 65 in bc33f4e

	def invalidate_volume(f):
	@wraps(f)
	def wrapper(*args, **kwargs):
	ret = f(*args, **kwargs)
	obj = args[0]
	if isinstance(obj.transfer_function, ProjectionTransferFunction):
	obj.sampler_type = "projection"
	obj._log_field = False
	obj._use_ghost_zones = False
	del obj.volume
	obj._volume_valid = False
	return ret
	return wrapper

but it sounds like with it in there other things were broken, so we're going to see what happens with the tests.

EDIT: All tests pass with just this one commit.

PR Checklist

• New features are documented, with docstrings and narrative docs
• Adds a test for any bugs fixed. Adds tests for new features.

[jzuhone]

This is ready to go. I see no reason why this change isn't the right one.