Changed default write disposition

[keegansmith21]

Default table load probably shouldn't be destructive (as I've discovered the hard way).

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.04 ⚠️

Comparison is base (1e16b5c) 87.77% compared to head (b3a5c03) 87.73%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #623      +/-   ##
===========================================
- Coverage    87.77%   87.73%   -0.04%     
===========================================
  Files           38       38              
  Lines         5169     5169              
  Branches       623      623              
===========================================
- Hits          4537     4535       -2     
- Misses         496      501       +5     
+ Partials       136      133       -3     

Impacted Files	Coverage Δ
...ervatory-platform/observatory/platform/bigquery.py	89.23% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[jdddog]

Thanks Keegan, that looks like a safer option. I've updated a couple of workflows in Academic Observatory to work in line with this: The-Academic-Observatory/academic-observatory-workflows#170