L2 Stopwatch 8 should introduce computed property

[ybakos]

I believe Stopwatch Lesson 13 introduces optionals and computed properties, and students could be better prepared if we introduce a computed property in Lesson 8, to encapsulate planetarySystem.planets.count.

This requires changing Stopwatch Lesson08.pages, and updating tags 9 - 15 in SwiftEducation/SpaceAdventure.

[bishalg]

I believe as there will be many Lesson and Levels we should use consistence standard like - "< Level-Number > - < App-Name > - < Lesson-Number > "

Currently we are using only " < App - Name > " this can be confusing.

[ybakos]

@bishalg Do you mean using a standard way of referring to the lessons in our issues, so it is easier to locate? I agree, and I have updated this issue's title to reflect that. Let me know if this is not what you meant.

[bishalg]

Yes, exactly, also every where it is possible. Even in the repo name with "< Label-Number >" prefix would have been better. As we will have 100 of repo under name such as "UnitConverter" https://github.com/SwiftEducation/UnitConverter , but if its name was "L2-UnitConveter" we could easily know even from github repo which Lesson belongs to which level.