getScore_custom: getScore_custom
In mamouzgar/hsslda_dep: LDA with Hybrid Subset Selection.

getScore_custom: placeholder for a custom metric

1	getScore_custom(x, y, cols, custom.score.method, ...)

`x`	dataframe of training data
`y`	vector of class labels matching training data rows
`cols`	vector of column names
`custom.score.method`	a custom-function that takes in x, y, and cols, and outputs a score where the larger the value, the more optimal your separation criteria is.