Add flatten operation to flatten a set of points into a single point

[nathanielc]

Flatten a set of points on specific dimensions.
For example given two points:

m,host=A,port=80 bytes=3512
m,host=A,port=443 bytes=6723

Flattening the points on port would result in a single point:

m,host=A 80.bytes=3512,443.bytes=6723

       |flatten()
           .on('port')

If flattening on multiple dimensions the order is preserved:

m,host=A,port=80 bytes=3512
m,host=A,port=443 bytes=6723
m,host=B,port=443 bytes=7243

Flattening the points on host and port would result in a single point:

m A.80.bytes=3512,A.443.bytes=6723,B.443.bytes=7243

       |flatten()
           .on('host', 'port')

Since flattening points creates dynamically named fields in general it is expected
that the resultant data is passed to a UDF or similar for custom processing.

• Rebased/mergable
• Tests pass
• CHANGELOG.md updated