Abstract `SpecimenMap$map()` into separate base layer methods

[jasonratcliff]

Refactor public method map() into separate methods for each possible input in baselayer:

• map_base()
• map_ggmap()
• map_elevatr()

Design

• Define new active function cartography as container for plot base layer.

Unit Tests

• R Packages (2e) Testing Design

Reference

• Advanced R | R6 OOP

  • Active bindings

• Simple Features

  • R Package sf