made the header of the first constructor straight
added method setRowsNumber
changed elements_.resize(rownumber)
to setRowsNumber() call in the first constructor
added second constructor's declaration with row_number,
columns_number and a vector of initializers in the parameters
added setColumnsNumber method declaration
added method getColumnsNumber()
added getRowsNumber() method
added setElement() method
added getElement() method
added isElementIndexInRange() method to private part
added declarations for next methods:
- setElementsFromVector();
- getElementsToVector();
- operator* for one value, vector and another matrix;
- operator+ for another matrix;
- operator- for another matrix;
- operator== for another matrix;
added set3RowsForNewMatrix and TestCase tests