Climate: Add year counter to models

[#46377401]
Lab Grapher: fix use of dataSampleStart in addSample(s) functions

This allows dataSampleStart to actually be used correctly.
Climate: Plot graphs against year instead of ticks.

This is hacked using dataSampleStart = 2013 and 
sampleInterval = 1/ticksPerYear.

A better way would be to use addDataPoints instead of
addSamples. However, the currently implementation of
addDataPoints is broken if you try and graph multiple
lines at once.

[#53656699]
Climate: Fix step function