Integration with REXview

[agzimmerman]

REXview is a platform / App ecosystem to visualize 3D models in AR on Android and Apple smartphones and tablets. we want to create a interface to the Rex platform to directly upload models from a jupyter notebook. the envisaged workflow looks like this:

1. create an account on login.rex.cloud
2. create an api key
3. deposit the api key in the credential file
4. create a geomodel with gempy
5. call function to export to Rexfile and upload to the platform
6. display Rextag QRcode in the notebook
7. scan rextag with smartphone to place AR model

[agzimmerman]

This is being worked in PR #296