Inconsistencies between the definitions of the set functions

[hartig]

When working on PR #98 I noticed several inconsistencies between the definitions of the set functions.

1. Some definitions use "card[S]" to denote the cardinality of a sequence S (e.g., Sec.18.5.1.3 Sum) whereas others use "|S|" (e.g., Sec.18.5.1.7 GroupConcat). My proposal is to consistently use the latter.
2. Some definitions use the word "when" when capturing conditions (e.g., Sec.18.5.1.3 Sum) whereas others use "where" (e.g., Sec.18.5.1.4 Avg). My proposal is to consistently use the word "if".
3. Additionally, the definition of GroupConcat contains only the function signature without the scalarvals argument. A second function signature needs to be added that contains the scalarvals argument.

[hartig]

To avoid merge conflicts with the changes in PR #98 I will wait with addressing the points in this issue until we are done with PR #98.

[hartig]

Another inconsistency that I forgot to mention:
4. In some definitions, the indexes of elements in the given sequence start from 1 (e.g., Sec.18.5.1.3 Sum) whereas other definitions use indexes starting from 0 (e.g., Sec.18.5.1.7 GroupConcat). My proposal is to consistently use the former (i.e., starting from 1).

[hartig]

Closed by PR #108