Feature: Historical maps as anchored images

[janheinrichmerker]

Implementation details described in #5

[sushoff]

1. "feature matching" is a in many cases a library (no need to reinvent the wheel)
2. if so: the feature that the user is looking for could be either lines or dots/ stars of different size
3. in historical maps, frequently stars are drawn with "spikes" - in this case, the user needs to define what a star is, I'm afraid (provide the shape that the feature matching should look for)
4. in case of more recent/ mmodern maps, stars are drawn as "dots" - then, we can simply use a common "source extractor" (don't reinvent the wheel)

it would be really nice to have - but if not quickly doable, don't waste time!