Create geosoft_gxf_io.py

[ThomasMGeo]

GXF File Format IO is a common file type for grav/mag for USGS releases. It's an ASCII file type with variable header structures.

On the dataset that I tested this on, it is upside down in the decoding to xarray. This is not impossible to fix, but I do like keeping the option for numpy and metadata output.

I bet this could be optimized further, will try to find more datasets to test with (smaller the better!). Any comments/optimizations are welcome.

Relevant issues/PRs:: This addresses #538 that has some more information about the format itself.

[ThomasMGeo]

I found a small file that works from here: https://pubs.usgs.gov/of/2000/ofr-00-0198/html/wyoming.htm

Specifically: mag5001_gxf.gz | 2000-04-24 16:50 | 57K

Potential citation:

Kucks, R.P., and Hill, P.L., 2000, Wyoming aeromagnetic and gravity maps and data—A web site for distribution of data: U.S. Geological Survey Open-File Report 00-0198, https://pubs.usgs.gov/of/2000/ofr-00-0198/html/wyoming.htm

[ThomasMGeo]

ToDo: Licensing of data file from USGS.

[ThomasMGeo]

ToDo: Add Attribution to gist

[ThomasMGeo]

For USGS, looks like we just need to add attribution as well: https://www.usgs.gov/faqs/are-usgs-reportspublications-copyrighted

[ThomasMGeo]

@santisoler added some text about the USGS, the attribution to the gist was already there. Let me know if I need to do anything else! I probably won't edit this further until I have some more feedback.

[ThomasMGeo]

@santisoler , is there anything else you want to see?

[santisoler]

Hi @ThomasMGeo. Thanks for pushing these changes. Let me take a look at this and I'll come back to you.