Update old .ipynb tutorial notebooks

[kakirastern]

Update some old .ipynb tutorial notebooks.

Fixes #335.

[kakirastern]

Think I might need to turn off caching of downloaded files for the tutorials, and then add the comment that cache = False can be modified to cache = True if we do not want to see the timeout or URLErrors. Will need to go through the tutorials one-by-one for that.

[adrn]

I like the sentiment of the title, but I'm a little confused about the changes you've made: can you explain a bit? e.g., I don't think changing "HCG 7" to "hcg7" will fix the broader issues. I do, however, agree that we should review each notebook, identify places where they try to talk to the internet, and decide whether we want to keep those instances or replace them with cached values. I could imagine, for example, replacing:

hcg7_center = SkyCoord.from_name('hcg7')

with something like:

# This line accesses the internet - if you have an internet connection, try this out, 
# otherwise use the default cached values included below this line:
# hcg7_center = SkyCoord.from_name('hcg7')
hcg7_center = SkyCoord(ra=..., dec=...)

[kakirastern]

I originally thought it shouldn't make a difference between the queries "HCG 7" and "hcg7" too, but after testing a bit I found that somehow Travis CI thinks it does... (Maybe a long time ago there used to be no difference?) If I don't change this occasionally an error would be thrown due to the syntax. I will add the few lines of explanation you gave in #341 (comment) however, to clarify the situation a bit more. I will test it again locally as well to confirm.

[adrn]

@kakirastern I think the "HCG 7" issue is a red herring: try it locally, you'll see that the name "HCG 7" resolves just fine

With astropy v3.1:

>>> from astropy.coordinates import SkyCoord
>>> SkyCoord.from_name("HCG 7")
<SkyCoord (ICRS): (ra, dec) in deg
    (9.81625, 0.88805556)>

I'm going to have a go at making some changes to solve #348, so this might not get merged in the end, but I'll keep this open for now. Thanks!

[kakirastern]

Yup, no problem @adrn... Have updated #341 (comment) accordingly.

[kakirastern]

I think the "HCG 7" issue is a red herring: try it locally, you'll see that the name "HCG 7" resolves just fine

Yes, it is strange. I did try it locally and I remember the term "HCG 7" did resolve just fine... Not sure why Circle CI is getting so sensitive.

[eblur]

I'm closing this PR because the original issue was handled in PR #370