Create new gallery category "Histogram"

[michaelgrund]

Description of the desired feature

As discussed in #876 and mentioned in #794 (rose wrapper) we should add a new category "Histogram" to the gallery in which we store e.g. the rose and histogram (#1072) examples.

Are you willing to help implement and maintain this feature? Yes, but volunteers are welcome.

[seisman]

To create the new "histogram" gallery category, you need to:

• create a directory examples/gallery/histograms
• Add a README.txt in the directory examples/gallery/histograms
• Add the path to the conf.py file
• Move the example rose.py to the directory examples/gallery/histograms

See #1019 for an example PR.