New module wfdb.io._url.

This module contains functions and classes for accessing remote files
via HTTP.

The RangeTransfer class provides an efficient interface for requesting
and retrieving a range of bytes from a remote file, without knowing in
advance if the server supports random access.

The NetFile class implements a standard "buffered binary file"
(BufferedIOBase) object that is accessed via HTTP.  This class
implements its own buffering, rather than implementing the RawIOBase
API and using a BufferedReader, because the buffering behavior depends
on the server (if the server doesn't support random access, we want to
buffer the entire file at once.)  We also want to enable a mode where
the entire file is explicitly buffered in advance, which may be more
efficient when the caller intends to read the entire file.

The openurl function provides an open-like API for creating either a
text or binary file object.

Author: Benjamin Moody