colorbar: ax instead of ax1 & ax2

[mathause]

• [x] Closes #104
• [x] Tests added
• [x] Passes isort . && black . && flake8
• [ ] Fully documented, including CHANGELOG.md

That was more difficult than I anticipated. However, (apart from the deprecation), the logic is simplified and more similar to what matplotlib does. Docu is still missing...

f, axs = plt.subplots(...)

h = axs[0].pcolormesh(...)

mpu.colorbar(h, axs)