Rules element with Rules and Group by

[Rikkola]

Examples of the added feature are in yard-core/test/rules folder.

Few notes about this.

• This adds a Store type for YARD inputs. Other than that the input type is not really used ATM.
• Previously every type used in YARD elements and as YARD outputs was a boolean, number, string, map and list. This adds Date and Duration.
  • To get a date there is a date() function added to MVEL. Style borrowed from FEEL.
• sum and count as groupby accumulator functions. Sum also accepts parameters that can be resolved with MVEL.

Things to add later:

• Types DateTime, Time
• Functions dateTime(), time()
• Rest of the group by accumulate functions

Things to reconsider:

• I also automatically imported java.time.Duration. However Duration as a name is a likely input name. This might not be an issue and the user can work around it or we mask Duration and give it another name.

[lucamolteni]

This PR introduces a whole lot of Drools inside YaRD.
It does it directly, as its resulting syntax is very similar to Drools'.

The problem is that the current YaRD syntax is pretty different from Drools', as it's heavily inspired by DMN instead. So with this PR it feels like having Drools and DMN together in a single file, which seems a bit confusing.

For scorecards specifically I'm not sure we need this feature, so I wouldn't merge as it is. Perhaps I'm wrong and the users will need this from day one, but I still think we should design the YAML syntax first.

[lucamolteni]

If I understand correctly this is putting a whole Drools like syntax dialect inside the current AST of YaRD, which I'm not necessarily against but it seems very different to what we have now.

Currently there is no documentation nor specification of what YaRD should be so this seems like a risky change

[lucamolteni]

This (and its test) needs definitely a better name i.e.

MVELExpression
MVELThen