Factor support

[thomasp85]

Fixes #34

In order to avoid conversion of factors attribute elements are
initialised prior to assignment if the attribute does not exist or if
the assignment is overwriting all elements within that attribute

[thomasp85]

FWIW, the failed build has nothing to do with this PR...

[IgnatiusPang]

The codes look ok to me.

[mathlandry]

Are these changes present in the R igraph package? In my case, set_vertex_attr() and vertex_attr()<- both still convert factors to integers

[vtraag]

@mathlandry , no this code is not yet merged. This is a Pull Request with new proposed code that should still be integrated in the main codebase. After it will be merged, you can install it using remotes::install_github("igraph/rigraph@master"). I will try to do this shortly.

[krlmlr]

Thank you for your contribution. This has conflicts now and needs to be redone. The original issue #34 has a reprex and points to this PR.

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