Fix #714: Make TAGCOUNT always greater than the last instantiated Tag's count

[Technici4n]

This fixes #714:

     Testing Running tests...
Tag count of Dual{ForwardDiff.Tag{FDPrecompilationIssue.var"#3#4"{Int64}, Int64}}(0,1) is 1
Tag count of Dual{ForwardDiff.Tag{FDPrecompilationIssue.var"#1#2"{ForwardDiff.Dual{ForwardDiff.Tag{FDPrecompilationIssue.var"#3#4"{Int64}, Int64}, Int64, 1}}, ForwardDiff.Dual{ForwardDiff.Tag{FDPrecompilationIssue.var"#3#4"{Int64}, Int64}, Int64, 1}}}(Dual{ForwardDiff.Tag{FDPrecompilationIssue.var"#3#4"{Int64}, Int64}}(0,0),Dual{ForwardDiff.Tag{FDPrecompilationIssue.var"#3#4"{Int64}, Int64}}(0,2)) is 2
     Testing FDPrecompilationIssue tests passed

This adds an atomic operation for each Tag instantiation. This might be problematic when computing many fast derivatives in a tight loop?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.76%. Comparing base (c310fb5) to head (4cb3336).
Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #724      +/-   ##
==========================================
- Coverage   89.57%   86.76%   -2.81%     
==========================================
  Files          11       10       -1     
  Lines         969      922      -47     
==========================================
- Hits          868      800      -68     
- Misses        101      122      +21     

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