Various improvements for visualizations

[timmysilv]

Context: Now that the visualization improvements have been added to MrMustard, there's some feedback that needs addressing. This PR does that.

Description of the Change:

• Change the default colour scheme in the 2D/3D Wigner plots from viridis to RdBu to distinguish from the density matrix plot as suggested in #418. I definitely agree with the choice - negative is red, zero is white, and positive is blue. Felt semi-intuitive. If you'd prefer another one from plotly's list, lmk!
• Force the mid-point of the colour axis in those Wigner plots to always be 0 instead of auto-scaling, and show the scale
• Change the widget in the Fock repr to be kinda-square
• Change the 2D Fock magnitude plot to be a bar graph

Benefits:

• Things look more how we want them to
• Wigner color scale is consistent between plots to avoid confusion, and scale is shown now
• Fock magnitude plot no longer falsely suggest that it represents continuous information

Possible Drawbacks:

• Maybe you don't like RdBu?
• Maybe you'd prefer the colour scale to be hidden like before?

Related GitHub Issues: Fixes #417, Fixes #418

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.88%. Comparing base (ee63743) to head (2f5dda1).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #450 +/- ## ======================================== Coverage 88.88% 88.88% ======================================== Files 100 100 Lines 7020 7020 ======================================== Hits 6240 6240 Misses 780 780 ``` | [Files](https://app.codecov.io/gh/XanaduAI/MrMustard/pull/450?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XanaduAI) | Coverage Δ | | |---|---|---| | [mrmustard/lab\_dev/states/base.py](https://app.codecov.io/gh/XanaduAI/MrMustard/pull/450?src=pr&el=tree&filepath=mrmustard%2Flab_dev%2Fstates%2Fbase.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XanaduAI#diff-bXJtdXN0YXJkL2xhYl9kZXYvc3RhdGVzL2Jhc2UucHk=) | `96.19% <100.00%> (ø)` | | | [mrmustard/physics/fock.py](https://app.codecov.io/gh/XanaduAI/MrMustard/pull/450?src=pr&el=tree&filepath=mrmustard%2Fphysics%2Ffock.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XanaduAI#diff-bXJtdXN0YXJkL3BoeXNpY3MvZm9jay5weQ==) | `92.20% <ø> (ø)` | | | [mrmustard/widgets/\_\_init\_\_.py](https://app.codecov.io/gh/XanaduAI/MrMustard/pull/450?src=pr&el=tree&filepath=mrmustard%2Fwidgets%2F__init__.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XanaduAI#diff-bXJtdXN0YXJkL3dpZGdldHMvX19pbml0X18ucHk=) | `98.13% <100.00%> (ø)` | | ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/XanaduAI/MrMustard/pull/450?dropdown=coverage&src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XanaduAI). > **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XanaduAI) > `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` > Powered by [Codecov](https://app.codecov.io/gh/XanaduAI/MrMustard/pull/450?dropdown=coverage&src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XanaduAI). Last update [ee63743...2f5dda1](https://app.codecov.io/gh/XanaduAI/MrMustard/pull/450?dropdown=coverage&src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XanaduAI). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XanaduAI).