Mercurygui

This release focuses on cosmetic improvements, including dark mode support.

Changed:

- We now depend on our own fork of pyqtgraph `cx_pyqtgraph`.

Added:

- Support for dark interface themes, such as the dark mode in macOS Mojave. This will
require a version of PyQt / Qt which supports system themes, such as v5.12 for macOS.

Changed:

- Bumped mercuryitc driver requirements.
- `MercuryMonitorApp` must now be explicitly imported from main.
- Moved utils to submodule `pyqt_labutils`.

Fixed:

- Fixed a bug which would cause saving of log files to fail if no heater module is
connected.

Changed:

- Switched plotting to from Matplotlib to PyQtGraph for better performance.
- Switched to scientific spin boxes.

Added:

- Plot gasflow and heater output below temperature plot.