gnss.GNSS_SIDESHOW.get_site_lat_lon: dload site list file if not exist

[yunjunz]

Description of proposed changes

• bugfix while running view.py --gnss-source SIDESHOW --ref-gnss P225 on the first time, where the reference GNSS needs the site list file to extract its lat/lon location, but the site list file has not been downloaded yet, thus, causing the following error:

(insar) yunjunz:~/data/test/SanFranSenDT42/mintpy>$ view.py velocity.h5 velocity --show-gnss --gnss-src SIDESHOW  --ref-gnss P225 --gnss-comp enu2los --gnss-label
run view.py in MintPy version 1.6.1.post1, date 2024-08-09
input file is velocity file: /Users/yunjunz/data/test/SanFranSenDT42/mintpy/velocity.h5 in float32 format
file size in y/x: (510, 510)
input dataset: "['velocity']"
turning glob search OFF for velocity file
num of datasets in file velocity.h5: 5
datasets to exclude (0):
[]
datasets to display (1):
['velocity']
data   coverage in y/x: (0, 0, 510, 510)
subset coverage in y/x: (0, 0, 510, 510)
data   coverage in lat/lon: (-122.600833333, 38.1, -121.750833673, 37.25000034000001)
subset coverage in lat/lon: (-122.600833333, 38.1, -121.750833673, 37.25000034000001)
------------------------------------------------------------------------
colormap: jet
figure title: velocity
figure size : [7.5, 6.0]
read mask from file: maskTempCoh.h5
reading data ...
masking data
data    range: [-1.3753852, 1.003038] cm/year
display range: [-1.3753852, 1.003038] cm/year
display data in transparency: 1.0
plot in geo-coordinate
create directory: /Users/yunjunz/data/test/SanFranSenDT42/mintpy/GNSS-SIDESHOW
Traceback (most recent call last):
  File "/Users/yunjunz/tools/mambaforge/envs/insar/bin/view.py", line 8, in <module>
    sys.exit(main())
  File "/Users/yunjunz/tools/MintPy/src/mintpy/cli/view.py", line 184, in main
    obj.plot()
  File "/Users/yunjunz/tools/MintPy/src/mintpy/view.py", line 1677, in plot
    self = plot_slice(ax, data, self.atr, self)[1]
  File "/Users/yunjunz/tools/MintPy/src/mintpy/view.py", line 549, in plot_slice
    ref_site_lalo = gnss_obj.get_site_lat_lon()
  File "/Users/yunjunz/tools/MintPy/src/mintpy/objects/gnss.py", line 1087, in get_site_lat_lon
    with open(site_list_file) as site_list:
FileNotFoundError: [Errno 2] No such file or directory: 'table2.html'

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