MIP-689 Pass min_row_count in udfs

Created by: jassak

• Add new placeholder input type factory to UDF generator (see documentation).
• Add MIN_ROW_COUNT as special case of the above type, to allow this value to be passed in UDFs. This feature targets specifically Descriptive Statistics and Histograms, which are the only algorithms that must use this value internally (closes MIP-689).
• min_row_count is now equal to 1 in algorithm validation tests.