Start on coordinates APE
Abstract and terminology
Removed unnecessary "s
Terminology clarifications
Finalize initial APE content
changed coordinates APE to APE5
Final updates to APE5 draft, including date and PR link
Clarifications for the high-level coordinates based on comments by @nden
Spelling fixes
whitespace and spelling tweaks
Added brief mention of generalized WCS, based on conclusions from google hangout
Merge pull request #6 from eteq/coordinates

APE5: Coordinates Subpackage Plan