put ts in strict mode and fix

[jitsedesmet]

This PR doesn't really fix any issues but it prevents them from occurring. I enabled "strict": true, "strictFunctionTypes": false, "strictPropertyInitialization": false in the typescript config and fixed the problems this caused.

[coveralls]

Pull Request Test Coverage Report for Build 1167179902

• 19 of 21 (90.48%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 90.509%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/aggregators/Max.ts	2	3	66.67%
lib/aggregators/Min.ts	2	3	66.67%
<!--	Total:	19	21	90.48%	-->

Totals
Change from base Build 1166618006:	-0.1%
Covered Lines:	1505
Relevant Lines:	1629

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💛 - Coveralls

[jitsedesmet]

Btw, would like to ask you not to merge this yet. Even when you think it's ready :)

[jitsedesmet]

Alright, we should be able to merge this now.