Make a distinct API module for all request-based functionality

[Zeitsperre]

Summary

Miranda's API request modules should be collected into one submodule, e.g. miranda.fetch or miranda.remote, with accompanying submodules based on the source in which they are targeting (eccc, ecmwf, esgf, etc.).

Justification

Many of the libraries that are leveraged to perform these actions use specific configurations for things such as:

• Fetching credentials (.rc files in the ~/ drive or fed directly to the API request class instance)
• Naming files
• Asynchronous download support (built-in or only available via import multiprocessing)

These helper libraries are completely optional as of Miranda v0.5.0, and are only installed if explicitly requested. Since this is not the major focus of Miranda and other libraries might be more appropriate for the job in the future, it would be good to move these to a separate module that is not loaded when running import miranda (from miranda import fetch).