Finished file hierarchy implementation

[nipunramk]

One comment -- currently the file hierarchy is set so that it's by default global (cwd is "root"). We can change the cwd through the FileController.

[codecov-io]

Codecov Report

Merging #120 into master will decrease coverage by 4.46%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master     #120      +/-   ##
============================================
- Coverage     63.03%   58.57%   -4.47%     
  Complexity      271      271              
============================================
  Files            66       70       +4     
  Lines          1339     1441     +102     
  Branches        136      149      +13     
============================================
  Hits            844      844              
- Misses          466      568     +102     
  Partials         29       29

Impacted Files	Coverage Δ	Complexity Δ
...nd/postgres/controllers/LineageEdgeController.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...y/ground/postgres/controllers/GraphController.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...ound/postgres/controllers/StructureController.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...pp/edu/berkeley/ground/postgres/util/TreeNode.java	0% <0%> (ø)	0 <0> (?)
...es/app/edu/berkeley/ground/postgres/util/Tree.java	0% <0%> (ø)	0 <0> (?)
...ey/ground/postgres/controllers/NodeController.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...ey/ground/postgres/controllers/FileController.java	0% <0%> (ø)	0 <0> (?)
...ey/ground/postgres/controllers/EdgeController.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...berkeley/ground/postgres/util/SharedFileState.java	0% <0%> (ø)	0 <0> (?)
...d/postgres/controllers/LineageGraphController.java	0% <0%> (ø)	0 <0> (ø)	⬇️
... and 4 more

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