[R][feature] Aspect importance

Team Dalex,

As compared to the python implementation, it appears that R's aspect_importance does not seem to have some functionalities and non unified API:

1. Specifying correlation mechanism (depend_method in python API). Currently, only numeric columns are supported.
2. Functionality seems to be distributed between triplot,dalexExtra. It would be great to bring it under DALEX like python API does. Here are some thoughts:

# create explainer
exp = explain(model, data, y) 

# create aspects object/class instead of passing `variable_groups` to `triplot::predict_aspects`
# when `variable_groups` is specified, `depend_method` should be NULL
exp_asp = get_aspects(exp, depend_method = 'association', variable_groups)

# get variable groups at required cutpoint in hclust
# called 'group_variables' in triplot
vg = get_variable_groups(exp_asp, h = 1, n = 5) # either h or n

# get global aspect importance
# triplot has `predict_aspects` which seems like a local method, but actually it is global
# one among h, n and variable_groups should be provided
model_parts(exp_asp, type = "variable_importance", h, n, variable_groups) # should have print and plot methods

# get local aspect importance
# one among h, n and variable_groups should be provided
predict_parts(exp_asp, type = "shap", seed = 1, h, n, variable_groups, show_triplot = FALSE)

# This will keep the API consistent with python
# and the user need not worry calling `aspect_importance`, `predict_triplot` from different packages
# knowing DALEX's `predict_parts`, `model_parts` will suffice

I will be happy to contribute, let me know if a PR is welcome.

Start by comparing the Python aspect-importance implementation with R's aspect_importance documentation, then trace how triplot and DALEXtra currently divide the functionality. Map the proposed get_aspects, get_variable_groups, model_parts, and predict_parts API before deciding scope. Done should mean the R functionality is unified under DALEX with correlation mechanisms and global/local aspect-importance workflows.