subsample: Subsample a pixel_data object
In azvoleff/teamlucc: TEAM land use and cover change data processing toolkit

Subsample a pixel_data object

subsample(x, size, strata = "sources", type = "training", flag = TRUE,
  classes = levels(x@y))

## S4 method for signature 'pixel_data,numeric'
subsample(x, size, strata = "sources",
  type = "training", flag = TRUE, classes = levels(x@y))

`x`	a `pixel_data` object
`size`	either 1) a number from 0 to 1, indicating `size` is the fraction of pixels to sample, or 2) a number greater than 1, in which case `size` is the number of pixels to sample. Size applies per strata, if stratification is chosen.
`strata`	whether to draw samples from within individual classes, nested within source polygons (`strata='sources'`), or from within individual classes alone (`strata='classes'`)
`type`	whether to subsample training data (`type='training'`) or testing data (`type='testing'`). Whichever type is chosen, the other type will be left untouched (for example, if `type='testing'`, the training data will not be changed).
`flag`	whether to swap training flag on sampled data (for example, flag sampled training data as testing data, if `flag=TRUE` and `type='training'`) or remove sampled data from dataset entirely (`flag=FALSE`).
`classes`	specifies which classes to sample, defaults to all classes in `x`