legend_title for map_summary()

[martijn-bollen]

I would add a new argument legend_title to map_summary(), which allows users to provide a title of choice.

use cases:

• translate current legend titles in camtraptor:::.map_summary_legend_titles() to another language
• add one, or a combination of scientificName, lifeStage, age or sex to the default legend titles

[damianooldoni]

Indeed, @martijn-bollen, this will be a low hanging fruit. I will check this after the release of vesion 1.0. And it's something I surely will tackle while working on version 1.1.