Added General functionalities

[dlcole3]

Made several small updates including:

• updated examples
• allowed get_node_data to take a user-defined set of nodes
• allowed filter_nodes/filter_edges to take a function other than isless for filtering out nodes and edges
• add upstream_nodes and downstream_nodes for directed graphs to give the list of all upstream and downstream nodes

[codecov[bot]]

Codecov Report

Merging #38 (9cf3191) into main (169a9c0) will increase coverage by 0.08%. The diff coverage is 97.77%.

@@            Coverage Diff             @@
##             main      #38      +/-   ##
==========================================
+ Coverage   93.54%   93.63%   +0.08%     
==========================================
  Files           9        9              
  Lines        1705     1743      +38     
==========================================
+ Hits         1595     1632      +37     
- Misses        110      111       +1     

Impacted Files	Coverage Δ
src/DataGraphs.jl	100.00% <ø> (ø)
src/functions.jl	94.59% <96.96%> (+0.34%)	:arrow_up:
src/datadigraphs/utils.jl	97.21% <100.00%> (ø)
src/datagraphs/utils.jl	91.80% <100.00%> (ø)
src/interface.jl	97.76% <100.00%> (+0.08%)	:arrow_up:

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