Fix int overflow in MAP

[SkafteNicki]

What does this PR do?

Fixes #1594
A corner case in MAP metric where a big area is calculated and it then fails on a specific check comparison with an int because the value would overflow on comparison. Fixes by initializing as float.

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• Did you make sure to update the docs?
• Did you write any new necessary tests?

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[codecov]

Codecov Report

Merging #1607 (e9fc0f3) into master (632e8e6) will decrease coverage by 51%.
The diff coverage is 71%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1607     +/-   ##
========================================
- Coverage      88%     38%    -51%     
========================================
  Files         226     226             
  Lines       11992   12029     +37     
========================================
- Hits        10601    4559   -6042     
- Misses       1391    7470   +6079