Yh/multi color visualizer

[yijiangh]

Summary

This is a long overdue PR on the continuum and truss visualizers.

Highlights:

1. multi-cell color support for both visualizers
2. fix Visualizer not displaying supports for problems parsed from .inp files #137
3. fix Visualization of first documentation example #99

What type of change is this?

• Bug fix in a backwards-compatible manner.
• New feature in a backwards-compatible manner.
• Breaking change: bug fix or new feature that involve incompatible API changes.
• Other (e.g. doc update, configuration, etc)

Checklist

This PR is only related to visualizer, which is not covered by unit tests for now.

• I ran all tests on my computer and it's all green (i.e. ] test).
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if appropriate)

[yijiangh]

@mohamed82008 I am sorry that this commit history is messy. I started working on this a while back and got busy with other stuff. Recently, when I got back on this, I merged from master and kept developing. I am not sure how we should proceed on this? Should I fix conflicts locally and push directly to master?

[yijiangh]

Did a mega commit merge by first resetting and then rebase onto master as suggested by @mohamed82008. Ready to be merged after the checks are passed.

[mohamed82008]

Thanks Yijiang!