move MKD serialization downstream to IIF

[dehann]

also fixes tests, and xref:

• mv Serialization.jl downstream to IIF ApproxManifoldProducts.jl#116
• Serialization and Marshaling (again) DistributedFactorGraphs.jl#590

[codecov]

Codecov Report

Merging #1318 (c0f0f57) into master (bb75c95) will decrease coverage by 0.07%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1318      +/-   ##
==========================================
- Coverage   29.53%   29.46%   -0.08%     
==========================================
  Files          59       60       +1     
  Lines        4899     4911      +12     
==========================================
  Hits         1447     1447              
- Misses       3452     3464      +12     

Impacted Files	Coverage Δ
src/DispatchPackedConversions.jl	0.00% <ø> (ø)
src/IncrementalInference.jl	77.77% <ø> (ø)
src/SerializationMKD.jl	0.00% <0.00%> (ø)

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