Urban Cooling: display mortality risk results

[davemfish]

The valuation module generates a mortality risk raster for the urban cooling model's serviceshed. So far I did not summarize any results from that raster while working on #89 because I don't know how to interpret the data, and I have strong reservations about presenting it to a user without a very clear explanation of the method and some expression of the uncertainty.

[davemfish]

Also see comments in the ucm_valuation.py in mortality_risk_calculation. There is a discrepancy between the docstring and the code that we should fix.