Iono computation - addressing #135

[mgovorcin]

Changes:

1. Skip iono_computation if lower/upper band unwrapped interferogram array is empty, i.e. all zeros. Return iono as zeros array
2. Skip downloading and using water mask if the AOI request is out of SWBD data coverage. TODO: introduce new water masks as mentioned in New Ionosphere Updates do not always complete in Anchorage (Track 131) #135 : EXAMPLES: https://worldcover2021.esa.int/viewer or https://land.copernicus.eu/global/products/wb
3. fix few typos and add notes in the main regarding the esd and calculation of burst jumps

[cmarshak]

Looks great. Requested some small changes/clarification. Otherwise, let's merge soon so we can test.

[dbekaert]

@mgovorcin Should have meta data to the iono correction in gunw to track how it was produced

Water mask source : e.g none, gssh, …
Method: e.g none, framebased, subswath

Then we can always track it in the product. Please add in next pr