MP6 Fix CRS handling

[cforgaci]

This is an issue raised as part of ropensci/software-review#718

Issue raised by @mpadge:

I saw errors with both of these lines:

epsg_code <- 32635
delineate(city, river, crs = "EPSG:31600")  # National projected CRS
delineate(city, river, crs = epsg_code)

There seems to be something wrong, or at least not sufficiently robust or general, with your as_crs() function. I would expect to be able to submit:

• An integer(ish), maybe with a warning if the rounded integer version differed beyond a tolerance from the input.

• A string, either like the example above, or a full sf string

• A full sf::st_crs() object

But I couldn't get any of these to work?

MP6 Fix CRS handling (this will also be related to MP2, which might yield a way to specify a single CRS to be used throughout)

[cforgaci]

This seems to be solved with #409. I have also checked as_crs() with the different inputs provided by the reviewer, and they all seem to produce correct results.