calls to OpenAltimetry API time out

[JessicaS11]

Last week, OpenAltimetry transitioned their backend from its original domain to NSIDC and from grabbing data from a local server to the cloud-hosted copy of the data (ICESat-2 only so far). This has caused icepyx tests that utilize the OA programmatic API to start timing out and thus fail (see https://app.travis-ci.com/github/icesat2py/icepyx/builds/266433627 from #452).

[JessicaS11]

After communicating with NSIDC on this, there are a few open tickets to deal with issues surrounding the migration of OpenAltimetry to NSIDC. Fixing the programmatic API and related functionality is one of them. Currently open and near-merging PRs in icepyx (e.g. #451, #452) are not time critical, so we will wait a few days to see if the issue is resolved and we can put through a patch to fix the failing tests (i.e. update the base_url for programmatic requests) before merging those.