Added "current" directory to cf-standard-names Data directory. This is an
exact duplicate  of the most recent numbered version (currently 27). Each time
a new version of the standard name table is created the "current" directory
should be replaced with a copy of the latest version. The current directory
provides an unchanging URL for the latest standard name and area_types table
which allows for bookmarking in browsers, but most importantly, it is searched
for by the CF checker code.

On branch master
 Changes to be committed:

new file:   current/build/cf-standard-name-table.html
new file:   current/src/area-type-table.xml
new file:   current/src/cf-standard-name-table.xml
Corrected problem with duplicate alias entry for spectral_radiance in xml file.

 On branch master
modified:   27/src/cf-standard-name-table.xml
modified:   current/src/cf-standard-name-table.xml