Use data minmax consistently by indicator

[corviday]

Adds an entry to the variable configuration files stating the expected minimum and maximum values of each variable, and uses those values as the minimum and maximum of the colour ramp for each variable across all datasets with that variable.

This allows a consistent comparison of map images between climatologies and timestamps within a climatology. Previously, the minimum and maximum of the colour ramp were chosen for each image on an ad-hoc basis.

Resolves #68

[corviday]

Demo

Comparing the live SCIP to the demo, there should be two observable improvements:

• when you select an indicator on the right and step between climatologies on the map (outermost set of buttons under the map), the upper and lower bounds of the colour map should not change (and they should mostly be roundish numbers)
• when you select a monthly or daily indicator and step between timestamps on the map (innermost set of buttons under the map), the scales should be better-chosen, that is, June on temperature scales should not look all red on the map. This change mostly affects temperature indicators; the others didn't tend to exceed their ad-hoc maximums the same way.