Default temperature units should be Kelvin rather than Celsius

**Is your feature request related to a problem? Please describe.**
In the current setup of the Virtual Rainforest all microclimate calculations are performed in Kelvin, and the results are then converted into Celsius units to be stored in the data object. Then when the `soil` and `animal` module use this data they convert it back to Kelvin units. This doesn't seem like a sensible thing to be doing.

**Describe the solution you'd like**
If we switched to using Kelvin as the default for data we could massively minimise the number of required conversions. We would basically only have to convert temperature data when it is input, for some of the functions of the plant module, and for plotting. The plant model uses both Celsius and Kelvin units so it will have to make conversions regardless of the temperature unit chosen

Start by tracing how temperature data moves through the microclimate calculations and data object, then inspect its use in the soil, animal, plant, and plotting modules. Done means the default stored temperature unit is Kelvin and conversions remain only at the input, plant-function, and plotting boundaries described in the issue.

Written by the indexing model from the issue text.