Add GPU memory release method

surfplot is such a excellect toolbox for visualization surface in Python.
However, I found that if I called it looping to draw, it could cause a GPU memory overflow (I only have 4G GPU mem) and IPython to crash. I found that I could manually clear the existing plotter to avoid this problem, while the surfplot only return plotter.show() not plotter.
So, I use
```
from brainspace.plotting.base import Plotter
Plotter.close_all()
```
to avoid GPU memory overflow.
I believe it can be better if there are some method that can automatically close plotter afer we build the matplotlib object.
I hope this can be helpful.