Fix an error and reorder some functions in the MCMCChain extension

[sunxd3]

error is: vn is not an argument to function _check_varname_indexing

also reorder some functions for readability

[coveralls]

Pull Request Test Coverage Report for Build 10699843039

Details

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage remained the same at 77.508%

---

Files with Coverage Reduction	New Missed Lines	%
ext/DynamicPPLMCMCChainsExt.jl	1	81.82%

Totals
Change from base Build 10672167384:	0.0%
Covered Lines:	2712
Relevant Lines:	3499

---

💛 - Coveralls

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.66%. Comparing base (3ffb003) to head (e449873).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #650      +/-   ##
==========================================
+ Coverage   75.95%   76.66%   +0.71%     
==========================================
  Files          29       29              
  Lines        3518     3518              
==========================================
+ Hits         2672     2697      +25     
+ Misses        846      821      -25     

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[mhauru]

LGTM