Adding rule 6.1.8 regarding getters and setters

[orchestr7]

What's done:

1. Rule that prohibits custom getters/setters for properties

[codecov]

Codecov Report

Merging #518 (160ab75) into master (babf843) will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #518      +/-   ##
============================================
+ Coverage     81.78%   81.89%   +0.11%     
- Complexity     1555     1559       +4     
============================================
  Files            73       74       +1     
  Lines          3897     3911      +14     
  Branches       1244     1249       +5     
============================================
+ Hits           3187     3203      +16     
  Misses          211      211              
+ Partials        499      497       -2     

Flag	Coverage Δ	Complexity Δ
unittests	81.89% <100.00%> (+0.11%)	0.00 <8.00> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ	Complexity Δ
...at-rules/src/main/kotlin/generated/WarningNames.kt	0.00% <ø> (ø)	0.00 <0.00> (ø)
...tlin/org/cqfn/diktat/ruleset/constants/Warnings.kt	98.26% <100.00%> (+0.01%)	10.00 <0.00> (ø)
...qfn/diktat/ruleset/rules/CustomGetterSetterRule.kt	100.00% <100.00%> (ø)	8.00 <8.00> (?)
...cqfn/diktat/ruleset/rules/DiktatRuleSetProvider.kt	97.10% <100.00%> (+0.04%)	8.00 <0.00> (ø)
.../cqfn/diktat/ruleset/rules/WhenMustHaveElseRule.kt	83.33% <0.00%> (+4.95%)	14.00% <0.00%> (-4.00%)	⬆️

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