Update srtm.py

prevent from bug when nx,ny fall on the see
Added the London Tube stations example.
MAINT: Reverted to using previous freetype version
Comment explaining the change
Supported SRTM results over the ocean.
Merge pull request #442 from pelson/srtm_ocean

Srtm ocean
Merge pull request #444 from cpelley/change_freetype

MAINT: Reverted to using previous freetype version
Review actions.
DOC: Fixed whatsnew references
Changes based on review
Front page tweak.
Removed the img_tiles documentation file.
Merge pull request #446 from pelson/final_v0.11_tweaks

Final v0.11 tweaks
Updated version switch and copyright year.
Added date of release.
Merge pull request #443 from pelson/high_resolution_imagery

Added the London Tube stations example.
BUG: Whats new plotting directive syntax
fix Img.world_files

testing under Python 3, the doctest hangs because of the `map` usage

(`map` returns a lazy "map object", and extending it's argument with its result
is a problem)

this was missed in the recent partial Python3 port

using a list comprehension seems nicer than forcing the map object to evaluate
with

    list(map(_convert_basename, result))
Merge pull request #448 from njwilson23/fix-world-files-doctest

fix Img.world_files
Merge pull request #447 from cpelley/bug_whatsnew

BUG: Whats new plotting directive syntax