Refactor all .sits_parallel_map calls to a single internal function

[gilbertocamara]

Describe the requested improvement Currently, there are many sits functions that use .sits_parallel_map() to execute a function in parallel over a set of blocks. These include sits_classify(), sits_apply(), sits_smooth(), sits_label_classification(), sits_uncertainty(), and sits_combine_predictions(). All of these function use very similar code, which is repeated inside them. It is advisable to reduce this duplication by using a single function to be called by all these high-level methods.

Associated sits API function .sits_parallel_map()

Additional context Add any other context about the improvement here.