Using a dedicated currency object instead of float

[oxisto]

Otherwise we have weird rounding errors. Fixes #254

[codecov]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (e1dfcb8) 86.02% compared to head (228ff13) 86.16%.

Files	Patch %	Lines
gen/portfolio.go	90.32%	3 Missing ⚠️
gen/portfolio_sql.go	80.00%	3 Missing ⚠️
service/portfolio/portfolio.go	60.00%	1 Missing and 1 partial ⚠️
service/securities/securities.go	50.00%	1 Missing and 1 partial ⚠️
gen/securities_sql.go	94.11%	1 Missing ⚠️
service/securities/quote_provider_ing.go	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #255      +/-   ##
==========================================
+ Coverage   86.02%   86.16%   +0.13%     
==========================================
  Files          20       20              
  Lines         959     1019      +60     
==========================================
+ Hits          825      878      +53     
- Misses         92       97       +5     
- Partials       42       44       +2     

Flag	Coverage Δ
unittests	86.16% <91.48%> (+0.13%)	⬆️

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