Axis unit labels

[esc24]

Whilst tidying up the derived phenomena example I noticed what appeared to be an unnecessary assignment to the derived cube's units. It was already in kelvin (as calculated by the product of two cubes), but there was an explicit assignment of units i.e.

air_temperature = exner_pressure * air_potential_temperature
# set phenomenon definition and unit
air_temperature.standard_name = 'air_temperature'
air_temperature.units = 'K'

Removing this leaves the unit as iris.unit.Unit('kelvin') since:

>>> iris.unit.Unit('K') * iris.unit.Unit('1')
Unit('kelvin')

so the assignment appears to be there to change the label not the physical unit. I believe this should not be used in an example:

1. it implies Iris does not propagate the unit
2. it shows assignment to units without caution regarding the existing unit and the values of the data

This PR modifies quickplot axis labelling to remove '1' e.g. when plotting model level number one now gets 'Model level number' rather than 'Model level number / 1', and makes use of iris.unit.Unit.symbol when shorter than the full unit string e.g. to avoid 'kelvin' replacing 'K'. One downside is the abbreviation of units such as 'days' to 'd'.

[pelson]

I think this is fine. IMHO pending the blocker #119.

[rhattersley]

Very nice. :-)

Since you need to rebase anyway ... would you consider putting the cherry on top of the icing on top of the cake and have some code to handle "d" => "days", "h" => "hours"?

[esc24]

Looking into it now.

[esc24]

All done. Squash or not?

[rhattersley]

Extra-super loveliness. Thanks @esc24 :-)

As for the squash - I think it's fine as it is.