New functions and function improvements

[sjevelazco]

• Implement null models.
• Implement a function for calculating variable importance.
• Implement ensemble of ESM.
• Create a new function or adapt correct_colivar() correction of collinearity based on points and not only on rasters.
• #335
• #312
• Write a code to calculate a default value for the parameter size in the net method. It must be implemented in fit_net. See the structure of 'mtry' argument in fit_raf function to guide you. (example https://stats.stackexchange.com/questions/181/how-to-choose-the-number-of-hidden-layers-and-nodes-in-a-feedforward-neural-netw#:~:text=However%2C%20neural%20networks%20with%20two,more%20than%20two%20hidden%20layers.)
• improve column names of block_partition
• add for all fit_ functions the additional arguments related to hyperparameters of model technique. All of them will be by default NULL, however with these arguments will be accessible to users who manipulate them
• add a column of algorithm names for all fit_ and tune_ function
• substitute threshold names 'specific' by 'sensitivity' and set it as default 0.9 - [ ] add tryCatch in all fit_ function, it was just implemented in fit_svm and fit_mx
• create a function for extracting raster data based on species points
• improve name of this function (partition_block / partition_standar / partition_skm - spatial k-mean)
• create a function for returning a table with model performance from a list of models fitted with fit_ and tune_ . For instance, table_perf(x = list(m1, m2, m3)).
• Think about how to improve column names for hyperparameters returned by tune_ function family.
• Explore the possibility of use fit_ within tuen_ in order to shorten tune_ family codes
• work in env_filtering
• when turn this repo open remover codecov TOKEN from "secret" repository section and from flexsdm/codecov.yml

[flee598]

Hi, first, thank you for the fantastic package, I am finding it very useful!
I came here to request/suggest the addition of a function to calculate variable importance, I see it is already on the list above, so I just wanted to say thank you put my vote towards such a function. Cheers, Finn.