added test SomeRandomElementsInMatrix3x4 into TestMatrixSetElement
added test SomeRandomElementsInMatrix3x4) into TestMatrixGetElement
added definition for setColumnsNumber() method
added rows_number_ and columns_number properties
changed getColumnsNumber() method, now it returns
columns_number_ property's value
changed setRowsNumber() method: optimised with using rows_number_ property
getRowsNumber() method now returns rows_number_ property
default constructor now sets rows_number_ and columns_number_ to zero and
uses setRowsNumber() and setColumnsNumber() methods
added tests set3ColumnsForMatrix2x2 and
set3ColumnsForMatrix2x2WithNotZeroElements to TestMatrixSetColumnsNumber