Fix incorrect usage of `percent'.

[vikasgorur]

Rename the argument to various sampling methods in TypedPipe and RichPipe
to 'fraction' instead of 'percent' as they accept values in the range [0, 1]
and not [1, 100].

Fixes #1448

[vikasgorur]

Would it be appropriate for these methods to throw an IllegalArgumentException if the fraction is not in the range [0, 1]?

Motivation: I ran a job input.sample(10).save(output) intending to copy a fraction of the data but didn't realize my mistake until the job finished running. An early failure might be helpful in such cases.

[johnynek]

Thanks. Yes please add:

require(0.0 <= fraction && fraction <=1.0, s"got $fraction which is an invalid fraction")

[johnynek]

Thank you!