Intermediate cache clearing

[jayqi]

Addresses #12

• Adds method hasCompletedAllDownstreamModules to WorkflowInterface and DAGWorkflow classes to check whether a given module's downstream modules have all completed.
• Adds intermediate cache removal to Execute. Upon completion of a module, it will evaluate hasCompletedAllDownstreamModules on all its upstream modules and remove the upstream module's cache if true.

[codecov-io]

Codecov Report

Merging #17 into master will increase coverage by 0.42%. The diff coverage is 84%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
+ Coverage   82.45%   82.88%   +0.42%     
==========================================
  Files          14       14              
  Lines         610      625      +15     
==========================================
+ Hits          503      518      +15     
  Misses        107      107

Impacted Files	Coverage Δ
R/workflow_interface.R	11.11% <0%> (-0.66%)	:arrow_down:
R/execution.R	96.38% <100%> (+0.33%)	:arrow_up:
R/workflow_dag.R	92.09% <82.35%> (+0.74%)	:arrow_up:

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[jayqi]

Note that this implementation may sometimes fail to clear a module's cache during execution. For example, suppose you had this workflow:

A -> B -> C
 ↘
  D

If module C completes, then it will clear module B's cache. But if this happens before module D completes, then hasCompletedAllDownstreamModules(A) will always evaluate to FALSE because B has already been cleared.

This is because module$hasCompleted() checks for the existence of the cache to determine completion. We'd need to maintain global states of completion at the workflow level in order to have complete, correct information.

[jayqi]

@cwschultz88 in case you missed it. ^

[jameslamb]

@jayqi I really appreciate the way you ASCII-arted that example. Super clear