Add the ability to set the number of cores for parallel learning methods

[taavi223]

• Allow user to specify all available cores by passing in 0, null, or just not specifying a number
• Allow user to specify a specific number of cores by passing in a number >= 1 (up to the number of available cores)
• Allow user to specify a percentage of cores by passing in a number x between (0, 1), in which case we will multiply the number of available cores by x, rounding to the nearest integer (but not rounding down to 0 or up to the number of available cores).