implement support for function atan2

[Dimitar Misev]

rasdaman should support the atan2 function (and allow alias arctan2 for it).

Properties:

• two scalar or array parameters, of same base type, which must be float or double (so if someone invokes with char it throws an error)
• result is same type as the inputs (float or double)
• uses ​std::atan2 for implementation

One can follow (approximately) the implementation of the mod function for example, as a guide on how to implement atan2.

Start from the lexer lex.ll and parser oql.yy, then add the implementation in qtbinaryinduce.hh/cc after QtMod. Also add tests in systemtest/00-select, and document ​here.