Improve plotting

[dbrakenhoff]

Create plotting submodule:

• Move plot.py functions into submodule
• Separate nlmod-style plotting and flopy-style plotting (see more info below)
• Move utility functions into plotutil.py
• Add map_array functions for quickly drawing results on a map w colorbar etc.
• Add animate_map flopy version
• remove deprecated plot methods

nlmod vs flopy style plotting:

• nlmod-style plotting is based on a model xarray.Dataset, and all plot methods should obtain grid information and any other details from that object.
• flopy-style plotting uses arrays and the groundwater model object to create plots

The following examples yield the same result.

Example nlmod-style plotting, based on model dataset:

import nlmod

# given some model data set `ds` 

# load heads
da = nlmod.gwf.output.get_heads_da(ds=ds)

# plot heads
ax0 = nlmod.plot.map_array(da, ds)

Example of flopy-style plotting based on groundwater flow model:

import nlmod

# given some groundwater flow model `gwf`

# load heads
hobj = gwf.output.heads()
h = hobj.get_alldata()[0]

# plot heads
ax0 = nlmod.plot.flopy.map_array(h, gwf)