Issue/323 add line

[TatianaBurek]

Pull Request Testing

• Describe testing already performed for these changes:

• Tested on dakota http://dakota.rap.ucar.edu:8080/metviewer_dev/metviewer1.jsp

• Recommend testing for the reviewer(s) to perform, including the location of input datasets, and any additional instructions:

• create a line plot using Python and mv_dev_python database

• add a line(s) using a new UI - buttons on the bottom of the page - Add line and Remove line

• using menu in the popup choose line type and position

• configure the color and line width

• create a plot and make sure that the lines(s) are present

• perform the same steps using other plots (except Perf, Taylor and Contour)

• Do these changes include sufficient documentation updates, ensuring that no errors or warnings exist in the build of the documentation? [No]

• Do these changes include sufficient testing updates? [No]

• Will this PR result in changes to the test suite? [No]
  
  If yes, describe the new output and/or changes to the existing output:
  

• Please complete this pull request review by [Fill in date].
  

Pull Request Checklist

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[JohnHalleyGotway]

I did some testing and this is mostly working well. Up to you if you'd like to merge as-is or address any of the details below.

Here's what does NOT work the way I'd like:

• Vertical lines for values that do NOT coincide with one of the independent variable values are not plotted (e.g. vertical line at 12 is plotted, but 13 is not).
• Adding vertical and horizontal lines is good. Would also be nice to add the one-to-one diagonal or perhaps other lines.
• I understand the color chooser selects lines from a rainbow. If possible, I'd prefer the reference lines be added with a default constant color (like gray) instead of the rainbow. But this is definitely a preference and not a requirement.

Here's what DOES work very well:

• I tested using this plot xml to create this image:
  plot_20220216_223421.xml.txt
  
• Added multiple horz and vertical lines.
• Selected multiple colors, line types, and thickness.
• Confirmed that adding/removing straight lines and derived lines works as expected.
• Confirmed that reloading the XML does work as expected.

Note that I only tested this using the series plot type.

[TatianaBurek]

I created METplotpy/viewer issues to address suggestions:
#375
dtcenter/METplotpy#198