Forms a real path from a base-path and a relative path.
For example,
'/directory/file1.txt' + 'file2.txt' -> '/directory/file2.txt'
'/directory/file1.txt' + '../file2.txt' -> '/file2.txt'
'/directory/subdir/' + 'file2.txt' -> '/directory/subdir/file2.txt'
'/directory/file1.txt' + 'subdir/' -> '/directory/subdir/'
Parameters: contextPath - Parameters: relativePath - |