Improve performance of `get_prop`

[serenity4]

Before this PR, accessing a property on a vertex or on an edge was quite slow:

julia> using MetaGraphs, BenchmarkTools

julia> g = MetaDiGraph(2)
{2, 0} directed Int64 metagraph with Float64 weights defined by :weight (default weight 1.0)

julia> set_prop!(g, 1, :name, "one")
true

julia> @btime get_prop($g, 1, :name)
  85.547 ns (4 allocations: 512 bytes)
"one"

This was due to a PropDict being constructed every time get_prop was called, even if the key existed. Deferring this creation only to cases where the key was not present offered a nice speedup:

julia> using MetaGraphs, BenchmarkTools

julia> g = MetaDiGraph(2)
{2, 0} directed Int64 metagraph with Float64 weights defined by :weight (default weight 1.0)

julia> set_prop!(g, 1, :name, "one")
true

julia> @btime get_prop($g, 1, :name)
  9.117 ns (0 allocations: 0 bytes)
"one"

I also addressed these same performance issues for the undirected MetaGraph and for edge properties.

[matbesancon]

I'll try to finish #124 to get GH actions and documentation again before merging this, but looks good

[matbesancon]

@serenity4 can you merge master into your branch? Then we can see if all good on CI side

[codecov[bot]]

Codecov Report

Merging #123 (a72e551) into master (f52cbe4) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #123   +/-   ##
=======================================
  Coverage   98.64%   98.64%           
=======================================
  Files           5        5           
  Lines         368      368           
=======================================
  Hits          363      363           
  Misses          5        5           

Impacted Files	Coverage Δ
src/MetaGraphs.jl	99.55% <100.00%> (ø)
src/metadigraph.jl	97.43% <100.00%> (ø)
src/metagraph.jl	97.72% <100.00%> (ø)

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