Humidity dependence of virus halflife & central heating option in calculator

Nicolas Mounetrequested to merge
feature/RH_dependent_virus_halflife into master

This includes a tooltip and user guide (thanks to @jdevine ).

The humidity is now an attribute of a Room object (with default value 50%).

Result do change significantly with central heating, when the ventilation is poor.

Closes #148 (closed)

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