Fix WorldBank urls and outdated data usage

[spjuhel]

Changes proposed in this PR:

• Remove the broken WORLD_BANK_WEALTH_ACC URL constant and FILE_WORLD_BANK_WEALTH_ACC filename constant
• Rewrite world_bank_wealth_account to fetch data via download_world_bank_indicator instead of downloading and unzipping a CSV file; interpolation/extrapolation logic and the no_land correction are unchanged
• Remove now-unused imports: zipfile, Path
• Update docstring year range for world_bank_wealth_account from 1995–2014 to 1995–2020 to reflect the current dataset coverage
• Update docstring and documentation to reflect change to 2019 real chained USD (from constant 2014)
• Update test values to reflect changes

This PR fixes #1291 and #1293

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• Documentation updated
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[spjuhel]

@emanuel-schmid or @peanutfun, this should be a quick one to review. And will fix some of the currently failing tests.

[carmensteinmann]

There are some administrative regions, which lead to an error when trying to create LitPop for them.
I started from an old, global LitPop object and wanted to create a new LitPop object for all region ids. The failing admin units are: ['ALA', 'ESH', 'HMD', 'PCN', 'SGS', 'UMI']
These are: Åland Islands, Western Sahara, Heard Island and McDonald Islands, Pitcairn Islands, South Georgia and the South Sandwich Islands, United States Minor Outlying Islands

Please find the code how to determine these below.

`country_names = global_litpop.gdf.region_id.unique()
iso3a_list = u_coord.country_to_iso(country_names, representation="alpha3")

info_ind = "NY.GDP.MKTP.CD"
data = []
network = []

for cntry_iso in iso3a_list:
try:
cntry_gdp = util_fin.download_world_bank_indicator(indicator=info_ind, country_code=cntry_iso, parse_dates=False)
except (ReadTimeout, RequestException):
print(f"Network issue: {cntry_iso}")
network.append(cntry_iso)
except ValueError as e:
print(f"Data/API issue for {cntry_iso}: {e}")
data.append(cntry_iso)

failing_c = []
for country in data:
print(country)
try:
exp = LitPop.from_countries(country, reference_year=2002)
except ValueError as e:
failing_c.append(country)
`