Add IERS-specific remote timeout - #5111

An alternative to my comment above: put the with ... set_temp... clause above all three of the existing download_file calls.

My main concern here is that someone else hacking on this might not understand why download_file is subtley different, but with the same name. (Yet a third solution, I suppose, is to change it to donwload_file_iers or similar.

None of that is a big deal, so if you like it as is, @taldcroft, I'm 👍 to merge this.

@eteq - I was on the fence about whether the name should be different, and even had def iers_download_file at one point. But as you say it doesn't make a huge difference here.

My version using the set_temp context manager has the slight advantage that in theory one could actually call it with a remote_timeout kwarg and that would get honored. In practice that probably won't happen, and the real the reason I did that was that was because I forgot about that option. In any case since you are good with merging, I think this is good enough.