Adds data cache

[Nowosad]

Related discussion - https://twitter.com/jakub_nowosad/status/1295030990121979908.

[kadyb]

1. There should be an argument cache in pointDTM_get() function. If retrieving all values fails, the function should return some data. Then that object can be passed back as a function argument and retrieved the rest of the data.

DTM = pointDTM_get(polygon, cache = NULL)
# partially a failure, try get again
DTM = pointDTM_get(polygon, cache = DTM)

[kadyb]

2. We should check if the files have been downloaded before. If so, they should be skipped when downloading.