Averages.meanValue(PTable<K, V extends Numeric>) implementation

Calculating the mean value for each key in a table is somewhat fiddly to implement
and users may also roll their own interpretations which perform badly (for example
by reducing twice or loading reduce set into memory). We will therefore provide a
convenient library implementation.
Added missing javadoc for Averages.meanValue
Added README entry for Averages.meanValue()
Merge pull request #1 from DavW/mean-average

Averages.meanValue(PTable<K, V extends Numeric>) implementation