version 1.3
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* [FIX] fiddled with imports and importsFrom to get stuff to work without conflict between igraph and raster.
* [ADD] added minus operator for popgraph objects to find differences in edge sets.
* [FIX] changed all *population_graph() functions to *popgraph() for simplicity.
* [FIX] fixed popgraph() to ignore issues where log(1-SRI) -> NaN due to very small values of SRI


version 1.2
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* [FIX] Changed geom_popgraph_edgeset() to geom_edgeset()
* [FIX] Changed geom_popgraph_nodeset() to geom_nodeset()
* [FIX] Changed decorate_popgraph() argument 'meta.stratum' to 'stratum'

version 1.1
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* [ADD] Added function geom_popgraph_edgeset() to plot popgraph directly in a ggplot()
* [ADD] Added function geom_popgraph_nodeset() to plot popgraph directly in a ggplot()
* [ADD] Added theme_empty() for plotting graphs without background axes.




