Validate that sort value is integer

[mmachatschek]

As described in our XSD-Schema, the value of the sort element has to be an integer.

[codecov-io]

Codecov Report

Merging #52 into develop will increase coverage by 0.13%.
The diff coverage is 100%.

@@              Coverage Diff              @@
##             develop      #52      +/-   ##
=============================================
+ Coverage      91.07%   91.21%   +0.13%     
- Complexity       181      185       +4     
=============================================
  Files             30       30              
  Lines            504      512       +8     
=============================================
+ Hits             459      467       +8     
  Misses            45       45

Impacted Files	Coverage Δ	Complexity Δ
src/FINDOLOGIC/Export/Data/Sort.php	100% <100%> (ø)	5 <4> (+4)	⬆️

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[howard]

LGTM!