Add FHM and TopoData.zip files to file download list

[volkcj]

This is an enhancement--
add existing model products to the dl list, e.g. FHM (FIS/HSI, NREI, and TopoData.zip).

[philipbaileynar]

This is a big task that will need to be addressed formerly. My vision is that the existing FTP download feature is deprecated and replaced with an API download feature that works much the same way. Obviously it would get files from the API instead of the FTP.

I like the idea of keeping the existing UI and replacing the file list on the left with the types of files that we know are available (those mentioned by @volkcj in the original issue, plus others). This doesn't guarantee that the files will exist for every visit, but it will be the files that we know about a priori.

The other way to do this is go visit by visit and make call(s) to the API to discover what files are available before showing the list to the user for their selection. I don't like this because the mere act of calling the API multiple times to find out what files are available will be time consuming for lots of visits. And not all visits will have the same files and so we would need some complicated UI for letting the user pick what they want to download.

I prefer the first option where the user picks a set of visits on the main grid and then selects one or more file types. They then get all the files of the selected types for the visits selected. There might be some nuances, such as Delft where we won't know ahead of time what all the discharges are and therefore which files to download.