Add support of colorbar

[zxdawn]

Add add_scale (plotting colorbar) to add_decorate.

[codecov]

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[coveralls]

Coverage increased (+0.004%) to 86.775% when pulling c208265 on zxdawn:add_scale into 3007391 on pytroll:master.

[djhoese]

@zxdawn This still needs a test, right? Or does this not work for you?

[zxdawn]

@djhoese I remember it works when using it alone. Will test again after my field-work.

[zxdawn]

Finally, I tested this and here's the example and result:

Script

import os, sys, glob
from satpy.scene import Scene
from satpy.resample import get_area_def
from satpy.utils import debug_on
from trollimage.colormap import greys, rainbow
import aggdraw

#debug_on()
#os.environ['PPP_CONFIG_DIR'] = '/datadir2/anaconda/miniconda3/source_codes/satpy/satpy/etc/'

date = '20190807'
save_dir = '/xin/figures/FY4A/scale/'
save_path = save_dir + date + '/'

os.makedirs(save_path, exist_ok = True)

# read data as usual
scn = Scene(glob.glob('/public/home/zhangxin/bigdata/FY4A/L1/DISK/20190807/FY4A-_AGRI--_N_DISK_1047E_L1-_FDI-_MULT_NOM_20190807060000_20190807061459_4000M_V0001.HDF'), reader='agri_l1')
channel = 'ir_br'
scn.load([channel])
img = scn.resample('LEKIMA_1km')

# set the right width and height for colorbar
height, width = img[channel].shape
h_legend = int(height/30)
w_legend = int(width/2)

# set the corresponding colormap ticks range
greys.set_range(-40 , 30)
rainbow.set_range(-90, -40)
rainbow.reverse()

# save datasets with decorate: scale
img.save_datasets(base_dir = save_path, filename='FY4A_scale.png',
                     compute = True,
                     datasets = [channel],
                     writer = 'simple_image',
                     decorate={'decorate': [
                     {'scale': {'colormap': rainbow, 'extend': False,
                                'width': w_legend, 'height': h_legend,
                                'tick_marks': 5, 'minor_tick_marks': 1,
                                'cursor': [0, 0], 'bg':'white',
                                # title settings
                                'title':'Brightness Temperature ($^\circ$C)',
                                'fontsize': h_legend*100, 'align': 'cc',
                                }},
                     {'scale': {'colormap': greys, 'extend': False,
                                'width': w_legend, 'height': h_legend,
                                'cursor': [w_legend, 0]
                                }},
                        ]
                     }
                 )

Result

Issues

1. I can't change the color of ticks and the background of label by "fill" = "white" in the scale.
2. There's a bug in pydecorate and I submit a PR: Add "check for font object" in "title" section pydecorate#9

[mraspaud]

LGTM. Is there any test you could add ? For example in this class:

satpy/satpy/tests/test_writers.py

Line 547 in 8237e1b

class TestOverlays(unittest.TestCase):

[mraspaud]

LGTM. Thanks for finishing this up.