Split the pyqtgraph-support code initially developed as part of
the main taurus project to create the taurus_pyqtgraph independent
module (hosted in its own repo) that can be loaded as a plugin by
taurus.

This repo starts off from the code as it was in commit 50456cc in
the taurus repo and extracts the pyqtgraph-related code and adapts
the setup.py and metadata files.
The history up to 50456cc can still be checked from the taurus repo,
but we squashed it into this initial commit to make the
taurus_pyqtgraph repo lighter.

Bellow is a log dumped from the taurus repo listing only the commits
related to pyqtgraph using the following commands on a taurus repo:

```
git checkout 50456cc
git log -- lib/taurus/qt/qtgui/extra_pyqtgraph lib/taurus/qt/qtgui/tpg

```

commit 9a72b8504a0763f11f7a972188650cfb0483434c
Author: cpascual <cpascual@cells.es>
Date:   Mon Dec 11 16:10:31 2017 +0100

    Fix Y2 setting by CurvesAppearanceChooser

    The implementation...