structure_(2|3)d_plotly enable gradient-colored bonds

[janosh]

• Add support for gradient bond coloring using interpolating between colors of connected atoms or custom color lists
• handles various color formats and interpolation
• add unit tests for various bond coloring strategies

example:

from matminer.datasets import load_dataset

import pymatviz as pmv
from pymatviz.enums import ElemColorScheme, Key, SiteCoords


df_phonons = load_dataset("matbench_phonons")

fig = pmv.structure_3d_plotly(
    df_phonons[Key.structure].iloc[4].make_supercell(2, in_place=False),
    elem_colors=ElemColorScheme.jmol,
    hover_text=SiteCoords.cartesian_fractional,
    show_bonds=True,
)
fig.show()

[codecov]

Codecov Report

Attention: Patch coverage is 91.83673% with 4 lines in your changes missing coverage. Please review.

Project coverage is 87.79%. Comparing base (b984d4a) to head (a5c4341).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pymatviz/structure_viz/helpers.py	93.75%	3 Missing ⚠️
pymatviz/structure_viz/plotly.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #280      +/-   ##
==========================================
+ Coverage   87.76%   87.79%   +0.02%     
==========================================
  Files          44       44              
  Lines        4651     4677      +26     
  Branches      893      899       +6     
==========================================
+ Hits         4082     4106      +24     
  Misses        358      358              
- Partials      211      213       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.