Add graph and matrix classes for xmap

[cynthiahqy]

• [x] add tests for xmap_to_matrix(), incl for dropping extra cols.

• [ ] read vignette from spdep again: https://r-spatial.github.io/spatialreg/articles/nb_igraph.html

• creation of spatial weight matrix is similar to forming xmap

[cynthiahqy]

• https://graphviz.gitlab.io/doc/info/lang.html for #48

[cynthiahqy]

Rather than classes, I think it makes the most sense to have a single xmap class (i.e. xmap_df), and everything else be "candidate links" -- which can be input into the as_xmap() generic. Candidate links can also be validated as crossmaps, without coercion, allowing users to validate transformation objects in existing workflows.

Coercion into xmap classes should be reserved for facilitating "advanced" or "extension" tasks --- i.e. visualisation, perturbation, multi-map transformations. Some xmap class variants could include "long" or "short" formats for ggplot; or recode/collapse variants which drop the weights columns (since it is implied).

[cynthiahqy]

Closing as separate issues will be created for necessary xmap variants and/or candidate link types