write a mergezones function

[santoshphilip]

The function would look something like this:

mergezones(idf, zonelist)

where zonelist looks like:

['zone1', 'zone2', 'zone3']

zone2 and zone3 are merged into zone1

[santoshphilip]

I have a working script for this, written because I needed it. This needs to be made into a function with pytests