pdrive windows should include slash

[veenstrajelmer]

Follow up from #1080
pdrive should be p:/ for windows (including slash), for observations.py and interpolate_grid2bnd.py. Although both now seem to work somehow. On linux, this is not required. The windows behaviour is to me unexpected and inconvenient, but it seems to be intended: https://stackoverflow.com/questions/23955968/windows-path-with-no-slash-after-drive-letter-and-colon-what-does-it-point-to

Behaviour on windows (excludes slash):

>>> from pathlib import Path; print(Path("p:","aa"))
p:aa

Behaviour on linux (includes slash):

>>> from pathlib import Path; print(Path("/p","aa"))
/p/aa