fix bug in greedy_heuristic_prioritization() feasibility calculations

[jeffreyhanson]

Fix bug in greedy_heuristic_algorithm() function so that it returns a solution if it is not possible to select enough planning units to meet the target for any species. Instead of throwing an error, it will now throw a warning and return a solution containing the cheapest set of planning units within the budget and locked out constraints. Note that this solution will have an objective value of zero, because it has zero probability of meeting any of the species' targets.

[codecov]

Codecov Report

Attention: Patch coverage is 55.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 92.24%. Comparing base (6fda197) to head (2661a1c).

Files	Patch %	Lines
R/greedy_heuristic_optimization.R	55.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
- Coverage   92.55%   92.24%   -0.32%     
==========================================
  Files          30       30              
  Lines        2592     2604      +12     
==========================================
+ Hits         2399     2402       +3     
- Misses        193      202       +9     

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