[ENH] added tick_labels, dpi and save_as functions to Plot

[eric2302]

Added three features:

1. tick_labels: Custom text on colorbars when calling Plot().add_layer(tick_labels=['text1', 'text2']), see for example plot below

2. dpi: Increase resolution of output figure when calling Plot().build(dpi=...)

3. save_as: Save figure when calling Plot().build(save_as=...)