plotspectra outfile and with_other_model opts

[sbailey]

This PR adds two backwards compatible options to plotspectra:

• outfile: specify the name of the output file instead of the default html_dir/title.html (I want to set the title in the plot to be more verbose than I want for the name of the file)
• with_other_model=True; set to False to turn off the other/default model loading

I'm not entirely sure I got the othermodel widget wrangling correct for all cases of the default templates vs. 2nd best fits vs. nothing, so please check how that was intended to work and that I didn't break anything.

[coveralls]

coverage: 2.01% (-0.004%) from 2.014%
when pulling 7ba0cd6 on plot_options
into a697b41 on main.

[weaverba137]

This seems straightforward to me, OK to merge.

[weaverba137]

Are there any issues that this PR closes?

[armengau]

@sbailey I changed the criterium to call viewer_widgets.add_model_select:
if (num_approx_fits is not None and num_approx_fits>0) or with_full_2ndfit
becomes
if with_other_model

Otherwise it would generate buggy pages in the case one plots (spectra only + standard templates)
(a situation which is not theoretical, eg blind VI of secondary targets)

[armengau]

I ran the tests (examples_prospect_pages.sh), and checked some of the html pages.
Also added a test with with_other_model=False
=> ok for me.

[sbailey]

Thanks for the review and the fixes. This PR doesn't address any pre-existing tickets; it was a spontaneous contribution.