Prepare version 2.18

[olebole]

Tasks:

• update version numbers
• write release documentation
• other release related documentation updates
• Merge Add numerical recipes to libxtools for Noirlab compatibility #387 and Allow empty IRAFARCH in irafpath() #388

Release notes will be automatically created by Gihub and extended by the introduction, including a rationale for the new major number:

NOIRLab recently published their own 2.18 version with the scope limited to the support of the Gemini data reduction pipeline. All NOIRLab changes are bugfixes, based on NOAO version 2.16.1. They were reviewed and (if useful) integrated into the IRAF baseline. This merge also triggers the update of the version number to be in sync with the NOIRlab version.

After release:

• Merge Update to 2.18 iraf-mac-build#18
• release the final 2.18-1 release of the macOS installer
• Update web page
• Update readthedocs