First step in adding calculation in Ampersand

[Michiel-s]

In relation to the discussion in #207, I propose to do a small experiment to see how we can add calculations to our expressions and what is needed for that.

Inspired by the SHACL constraint language for RDF (https://www.w3.org/TR/shacl/#core-components), we can define a list of calculations to support, like lessThan and lessThanOrEqual.

E.g. for lessThan we could add the operator <. We need to define for which types this is supported and the precedence of the operator. The types INTEGER, FLOAT, DATE and DATETIME.

RELATION r[A*Date]
RELATION s[A*Date]
REPRESENT Date TYPE DATE

RULE I[A] |- r < s

The meaning of the operator < would be the set of A*A for which the value of tgt of r is less than the value of tgt of s, with the restriction that r and s are univalent. So r < s is basically a calculated property for A.

The generated SQL can be a join of r and s (like with r;s) but instead of the WHERE clause stating r.tgt = s.src we need r.tgt < s.src.

What do you think @hanjoosten @sjcjoosten? Can we add this one to see how it goes?