Adds concurrent asynchronous downloading, fixes issue #19

[2320sharon]

This PR adds concurrent asynchronous downloading for models, and fixes issue #19. It takes the functionality within select_model_and_batch_process_folder.py and turns it into documented functions which you can find in file model_functions.py the scripts directory . Each of these functions has documentation that explains what the purpose it serves, what parameters it requires and what it returns. I've renamed some of the variables that were in the original select_model_and_batch_process_folder.py because the original names didn't describe the variable's meaning. The most important renames to be aware of are as follows:

W: weights_files
C: config_files
T: model_names

This PR adds dependencies: aiohttp, which are listed in the readme and yml files. I also reorganized the imports so most of the imports are at the top except the imports used by tensorflow-gpu used to check for GPUs.

I also updated the installation instructions in the readme.md file since installing the environment from the yml file was taking hours. I also updated the yml file with the new dependencies I added and included version numbers for some of the dependencies.

I'm happy to help with any questions or any problems that come from merging the code. I've tested this code on 3 different machines so I'm sure it will work.

[ebgoldstein]

Looks great @2320sharon !

[2320sharon]

Thank you @ebgoldstein !

[dbuscombe-usgs]

Thanks both!