re: changing the cover
The existing Excel file, used for demonstrating parametric insurance, provides an update to the main San Salvador exposures. It sets the value of each asset to a constant 500 USD and the cover to zero (in the main notebook the cover was undefined). It describes this as writing property-level insurance up to the level of 500 USD for each property.

(That's not how I would have implemented it: I would have left the asset values unchanged and set the cover to 500, but whatever, this PR is to fix the bug, not to change the analysis.)

In the past this must have worked, and my assumption is that when the notebook was created, CLIMADA treated 0 cover and NA cover as equivalent. Running the code now, it treats zero cover as exactly that: uninsured. I'm not very interested in finding out when and where that change was made, because I think this is now the correct behaviour. But it means that the Excel file needs to be updated to preserve its intended behaviour of generating an EP curve when each property is insured with 500 USD. That means setting the value and the cover of each asset to 500 USD.