Fixed plotting for mixscape.plot_barplot and sccoda.plot_effects_barplot

[Lilly-May]

PR Checklist

• Referenced issue is linked ( MIXSCAPE - ms.plot_barplot(mdata["rna"], guide_rna_column="NT") shows no graph output with papalexi_2021 provided data #659, How to plot the 0 value during different comparisons? #598, Wrong plot titles in scCODA part theislab/single-cell-best-practices#271 - I'll close the issues manually once this PR is merged, because I'd like to add one comment each to explain the changes to the users who opened the issue)

Description of changes

mixscape.plot_barplot issue
As reported in #659, the effects bar plot wasn't visible. Adding a simple plt.tight_layout() fixed the issue. I also resolved some warnings that occurred during plotting by using ax.set_xticks instead of ax.set_xticklabels.

sccoda.plot_effects_barplot
Reported in #598 and theislab/single-cell-best-practices#271.
These issues were basically reporting the same problem: there was a bug in the code that caused covariates with only zero effects to be filtered out of the plotting dataframe, even when the plot_zero_covariate parameter was set to True (which is the default). This also led to incorrect figure titles. I resolved the issue by removing the line of code causing the bug.

For these results:

So far, the generated plot looked like this (one plot missing, wrong titles):

NOW, i.e. after merging this PR, it will look like this:

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.56%. Comparing base (98e2bdb) to head (b8b3a78).
Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #667   +/-   ##
=======================================
  Coverage   65.56%   65.56%           
=======================================
  Files          47       47           
  Lines        6105     6105           
=======================================
  Hits         4003     4003           
  Misses       2102     2102           

Files with missing lines	Coverage Δ
pertpy/tools/_coda/_base_coda.py	56.30% <ø> (ø)
pertpy/tools/_mixscape.py	79.12% <ø> (ø)