error in bc.pl.riverplot_2categories() figsize param with besca253

Hi,
when using the latest conda environment from @schmucr1 `besca253_240415` I get the following error;

```
bc.pl.riverplot_2categories(adata, ['celltype1', 'celltype2'], figsize=(1400,600))
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
Cell In[7], line 1
----> 1 bc.pl.riverplot_2categories(adata, ['celltype1', 'celltype2'], figsize=(1400,600))

File /projects/site/pred/scseq/conda/envs/besca253_240415/lib/python3.9/site-packages/besca/pl/_riverplot.py:103, in riverplot_2categories(adata, categories, palette, threshold, figsize)
101 fig = go.Figure(dict(data=[data]))
102 if figsize is not None:
--> 103 fig.set_figheight(figsize[1])
104 fig.set_figwidth(figsize[0])
105 return fig

AttributeError: 'Figure' object has no attribute 'set_figheight'
```

I don't get this error with previous conda environement despite the besca function not being updated lately. So I assume is due to a version change of some package dependency, maybe matplotlib?

Could you have a look?

Also, it would be good to indicate the units in the function description for figsize (is this in pixel, cm? etc) and add this param usage in some of the examples.

Thanks,
Llucia

Open besca/pl/_riverplot.py around line 103 and reproduce riverplot_2categories with figsize=(1400,600) in the besca253_240415 environment. Check the Figure dependency/version involved, then review the function description and examples. Done means the figsize call works, its units are documented, and an example shows its use.

Written by the indexing model from the issue text.