figure-fairplus directive dropped the figure sizing

[proccaserra]

allow attribute width or height to be set e.g.:

width: 800px

possibly modify to wrap the figure in a {dropdown} element.

:open:
```{figure} prov-dm-overview.png
---
width: 800px
name: prov-dm-overview
alt: Overview of the Provenance Data Model main classes
---
Overview of the Provenance Data Model main classes.
```