[fix] Only map nodes of the exact same type #71

[slarse]

Fix #71

This PR fixes the problem of array type references and "normal" type references being matched, which in turn can result in crashes when the merge tree is being built. The reason for this behavior is that 3DM merge has absolutely no notion of correct relationships between parent and child nodes. Thus, if nodes of mismatched type are mapped, then their children may be merged such that whichever type comes out triumphant from the merge suddenly has children of types (or with roles) that it can't actually have. Thus, building the merged spoon tree crashes.

This is a very conservative fix, and forces matches to only be between exactly the same type of node. This may cause issues elsewhere. I'm not sure. Only benchmarks can tell. I do think it is necessary, however, due to 3DM's ignorance of valid and invalid parent-child relationships.

[codecov-io]

Codecov Report

Merging #73 into master will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master      #73      +/-   ##
============================================
- Coverage     79.50%   79.43%   -0.07%     
  Complexity      301      301              
============================================
  Files            27       27              
  Lines          1244     1245       +1     
  Branches        196      197       +1     
============================================
  Hits            989      989              
  Misses          156      156              
- Partials         99      100       +1     

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/se/kth/spork/spoon/SpoonMapping.java	83.58% <100.00%> (+0.24%)	27.00 <0.00> (+1.00)
src/main/java/se/kth/spork/base3dm/Pcs.java	69.56% <0.00%> (-4.35%)	9.00% <0.00%> (-1.00%)

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