Remove plots and clean package

[noahrhodes]

• remove plots closes #46
• rename plot_vega to powerplot closes #42
• Removed plots plotting functions and files.
• Moved vega plotting functions to appropriate directories
• Made the remove_information! function accept an input for removal keys
• Moved component plots into their own functions. They are only plotted if the PowerModelDataFrame is nonempty for that component. This prevents comps like DC Lines from appearing in the legend when they don't exist in the data.

[codecov-commenter]

Codecov Report

Merging #54 (cc7c59e) into dev (b793497) will increase coverage by 11.45%. The diff coverage is 96.47%.

@@             Coverage Diff             @@
##              dev      #54       +/-   ##
===========================================
+ Coverage   85.24%   96.69%   +11.45%     
===========================================
  Files          16       12        -4     
  Lines         664      333      -331     
===========================================
- Hits          566      322      -244     
+ Misses         98       11       -87     

Impacted Files	Coverage Δ
src/PowerPlots.jl	100.00% <ø> (+8.33%)	:arrow_up:
src/core/data.jl	80.00% <ø> (+23.18%)	:arrow_up:
src/core/options.jl	100.00% <ø> (ø)
src/core/utils.jl	96.77% <ø> (ø)
src/core/configuration.jl	88.88% <88.88%> (ø)
src/plots/plot.jl	91.30% <91.30%> (ø)
src/layouts/layout_engines.jl	94.82% <94.64%> (+39.27%)	:arrow_up:
src/core/attribute_validation.jl	100.00% <100.00%> (ø)
src/graph/common.jl	100.00% <100.00%> (+4.76%)	:arrow_up:
src/layouts/common.jl	100.00% <100.00%> (+56.25%)	:arrow_up:
... and 4 more

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