Adapt to changes in the ADB ATO data source

[khaeru]

Closes #18.

In addition to the case mentioned in the issue description:

• "-", meaning presumably "no data".

…I also discovered:

• "41 226.2" —using a space as a thousands separator. The files as of 2023-04-17 contained only "," as a thousands separator.
• "10860 .6" —space before a decimal separator.
• "long ton" —apparently a unit descriptor copied into a data cell; sheet TAS-FRA-027 at Economy Code=MMR, TIME_PERIOD=2022. Note that this unit does not match the units annotation for the overall sheet.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.57%. Comparing base (d52312e) to head (1765e43).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
- Coverage   93.58%   93.57%   -0.01%     
==========================================
  Files          32       32              
  Lines        1184     1183       -1     
==========================================
- Hits         1108     1107       -1     
  Misses         76       76              

Files with missing lines	Coverage Δ
transport_data/adb/__init__.py	94.54% <100.00%> (+0.05%)	⬆️

... and 2 files with indirect coverage changes