Update plotting function

[sbfnk]

The matrix_plot function uses fields::image.plot which seems to have been deprecated so this should be updated.

Other considerations relating to this function:

• should we replace it with a ggplot2 equivalent, reducing the number of dependencies and thus partially addressing #124?
• should we make this a plot S3 function, which could potentially eventually live in https://github.com/socialcontactdata/contactmatrix where it could be used also for contact matrices generated by other packages?

pinging @Bisaloo @lwillem for thoughts/views.

[Bisaloo]

For me:

• Yes to using ggplot2, which offers users more customization options
• Yes to making it a plot() / autoplot() function. My suggestion would be to put it directly in https://github.com/socialcontactdata/contactmatrix since it's one of the remaining items on the roadmap

[lwillem]

While ggplot2 has its advantages, I would recommend a less invasive change by switching to the updated function fields::imagePlot in this instance. This way, we remain within the base R plotting context. I can have a look at this next week. Reviewing the dependencies is certainly worthwhile, as well as the discussion about a generic plot() function.

[sbfnk]

Sounds like the short-term fix is @lwillem's suggestion, whereas in the slightly longer term we want to farm this out to the contactmatrix package (which will be imported by socialmixr).

[lwillem]

I updated the plot_matrix function as discussed. See #133 for more information.

[sbfnk]

Fixed in #133.