Update visualization_functions.R

[ESCRI11]

Hello developer,

I noticed when using your library that the function plot_features(), yielded the following error:

Error in stats::model.frame.default(formula = data[feature_names[i], ] ~  : 
  invalid type (list) for variable 'data[feature_names[i], ]'

I must note that I am using the function import_radiomic_table() as my data source is none of the one supported. I have no data from other sources, so I am not sure if my changes may break data imported with the different importing functions. Nevertheless the proposed changes solve my issue and the boxplots are created with no problem.

Please let me know if this is useful for all the use cases or if the problem may be on the import_radiomic_table() function.

Xavier Escribà Montagut