BUG: several small bugs in doc/source/PPVCube.ipynb

[neutrinoceros]

Trying to run this notebook showed that it's not currently compatible with modern yt or astropy

#225 and #226 resolve the first couple issues, but there are more:

• ambiguous field errors (yt 4.1+)
• dirty outputs (deprecation warnings from Updated ppv_cube.py to avoid deprecated function 'update_all_headers' #225 and BUG: fix usage of deprecated (and removed) clobber argument from astropy #226 need to be cleaned up)

[neutrinoceros]

I fixed a couple more issues in #228 but it's not over. I think that remaining errors (YTAmbiguousDataType, AttributeError, incorrect unit parsing ...) would benefit from someone else's look (@jzuhone ? @brittonsmith ? @cphyc ?)

[jzuhone]

@neutrinoceros #187 is probably relevant (and should be finished and merged, tbh) but I haven't had a chance to check yet

[neutrinoceros]

Seems fair. If I get no complaints about #228 I intend to merge it a couple days, and I'll assume the rest of the issues will be fixed later.

[jzuhone]

@neutrinoceros FWIW I'm the one who developed PPVCube and the only one who knows anything about it :) so you're waiting on me for #228, I'll try today

[neutrinoceros]

Sounds good.