Add cdippy user-agent to data access requests

[haileyajohnson]

The cdippy library adds a custom header to any requests is makes directly (see #17 ), but cannot add it to any requests made using netCDF4.Dataset, which is all the the data access requests.

Requirements:

• allows custom user-agent on requests
• supports lazy reads
• returns a netcdf dataset-compatible object

Options:
1). get a patch made in netcdf-c that allows for custom headers in dap requests
2. replace netcdf4-python with pydap, which already allows for custom headers, and wrap the return object in a dataset-like wrapper.