lesson5.task1.plusMinus: consider allowing signed numbers.

[gagarski]

Currently, to strictly check input string for correctness a student has to perform two checks:

• try to call toInt() on number (to check format and integer range) and throw IllegalArgumentException if NumberFormatException happens;
• match with regex (something like [0-9]+) (to forbid signed numbers (+1, -7 etc.)), throw IAE if number does not match this regex.

Most students usually do not do at least one of those and it is rarely noted by teachers. A possible bug can only be found by random tests in some rare cases.

This can be solved two ways:

• provide some non random tests (like 1 + +3, 12 + -2, 121212121212121212121 + 3). (I'm not sure if JUnit's assertThrows will allow us to check if IAE but not NFE is thrown)
• consider the first two examples correct inputs (and everything accepted by .toInt()). (This way we have to update the model implementation since it tries to strictly check input string).

[mglukhikh]

I added assertThrows tests for this task & computeDeviceCells.