`DataColumnArithmetics.kt` inconsistencies

[Allex-Nik]

There are three things in DataColumnArithmetics.kt that need to be changed:

1. In the functions

public operator fun AnyCol.plus(str: String): DataColumn<String> = map { it.toString() + str }
// and
public operator fun ColumnReference<Any?>.plus(str: String): ColumnReference<String> = map { it.toString() + str }

if the column contains null values, these functions convert each null to the string "null" and concatenate the argument str with it. These functions should be changed in a way that the null values are preserved. That is, each null value in the original column results in a null value for the corresponding element in the resulting column.

2. The functions gt and lt are not defined for nullable types. They should also work on columns that contain null values.

3. We need to add to this file new functions: geq (greater than or equal to) and leq (less than or equal to).