It is necessary to cover .preload and save_mesh methods with autotests.

[Maikenov]

It is necessary to cover the preload function with autotests. These autotests should include:
argument validation - the function accepts only values ​​of the type indicated in its definition. If there is a mismatch, a compilation error is expected.
Checking for the value of the "mode" argument - the method should accept only the "identity", "shift_and_scale", "to_cube" arguments.
Also need to do tests to accept these values (modes) ​​with "space" instead of "_" when issue #173 is done.

Checking that the Save_mesh , when given an empty filepath (filepath = ""), creates a file in the directory anyway.