MAINT: changing servicetype image/spectrum to sia/ssa

[bsipocz]

Proof of concept to see if how proposed changes of pyvo would look in practice

(this is just sed-ing through the code, the narrative text may need some updates, too)

(Also, I wonder why we only have 'sia' but not 'sia1', as the latter would be the correct, non-ambiguous one, thus I'm opening an upstream, pyvo PR for that)

closes #170

[bsipocz]

This should be ready to go. The content still needs an upgrade to mention SIA2, but that is beyond the scope of this PR.

With this in, I think astropy/pyvo#449 should be given the green light and get merged.

[bsipocz]

If we bump the required pyvo version to 1.5.3, we will be able to update these examples to use 'sia1' vs 'sia', but overall that maybe too fresh of a release to require (it's not even released, but planned to be out in the early autumn)

[bsipocz]

@tomdonaldson - This is ready to go. CI is expected to fail due to some unrelated server issues at heasarc (CI was passing two weeks ago, and only doc changes have been made since then)

[tomdonaldson]

Thanks @bsipocz. This seems to have caught all occurrences!