Replace holes in shapes with thin line gaps

[beveradb]

e.g. this shape:

https://homearea.info/#%7B%22saved_date%22%3A%222019-11-04T23%3A54%3A16.039Z%22%2C%22targets%22%3A%5B%7B%22target%22%3A%2269%20Morrison%20Street%2C%20Edinburgh%2C%20UK%22%2C%22walking%22%3A%2220%22%2C%22cycling%22%3A0%2C%22bus%22%3A%2210%22%2C%22coach%22%3A0%2C%22train%22%3A0%2C%22driving%22%3A0%2C%22deprivation%22%3A%226%22%2C%22radius%22%3A%220.%22%2C%22minarea%22%3A%220.00001%22%2C%22simplify%22%3A%220.0001%22%2C%22buffer%22%3A%220.0001%22%7D%5D%7D

becomes this, once sent to rightmove / zoopla:

We should implement logic to detect these holes in the resulting polygon, and remove thin line sections from this polygon (much like the existing join_multi_with_connecting_lines method adds these lines) until the resulting polygon has no holes left.