Errors found in `functions`

[despadam]

See remaining issues from #251

Also see #269 about exercises on scopes.

[despadam]

In calculate basic area fix text description that we expect 6.0 instead of 6.00 because the test will check for 1 digit only.

[edoardob90]

In Exercise 2, we should specify that we are always dealing with floats, never integers. Fix the example line:

Example: length=2, width=3, unit="m" should return "60000.0 cm^2" (because 2m × 3m = 6m² = 60000cm²)

to

Example: length=2.0, width=3.0, unit="m" should return "60000.0 cm^2" (because 2m × 3m = 6m² = 60000cm²)

[despadam]

In solution calculate area with yards, feet etc, the wrong reference solution was printed.

[edoardob90]

Exercise 2, Part 3: add a test that checks if a wrong unit is passed.

[edoardob90]

In Exercise 2: in the text (part 2 and 3), make it clear that the default unit is always cm