Print models according to SMTLIB standard

[sankalpgambhir]

Minor fix, SMTLIB models are printed as s-lists as required by the standard.

SMTLIB Standard 2.6, Figure 3.9 / Page 47

As highlighted, eldarica does not print the parentheses, only the individual inner elements.

Old printing:

Small model

$ ./eld -hsmt -ssol modelTest.smt2 
sat
(define-fun P ((A Int)) Bool true)

Empty model

$ ./eld -hsmt -ssol modelTest.smt2 
sat

New Printing:

Small model

$ ./eld -hsmt -ssol modelTest.smt2 
sat
(
    (define-fun P ((A Int)) Bool true)
)

Empty model

$ ./eld -hsmt -ssol modelTest.smt2 
sat
(
)

z3 quickly for comparison:

$ z3 -smt2 modelTest.smt2
sat
(
  (define-fun P ((x!0 Int)) Bool
    (or (= x!0 0) (exists ((x!1 Int)) (! (= x!0 (+ 1 x!1)) :weight 0))))
)

Dicovered when the scala-smtlib parser was freaking out trying to read models from eldarica.

Changelog:

• added outer parentheses and indentation to model printing
• updated test answers accordingly (not full regeneration)
• yices url is changed to https://eldarica.org/yices for the tests, and a -c is added to its wget call so yices isn't downloaded again every time the tests are run.

[sankalpgambhir]

I will make a PR to rebase the scala-2.13 branch up to master after this is merged.

[sankalpgambhir]

I forgot that the tests have models as text in their answers. Almost done correcting them.

[pruemmer]

Great, thanks a lot for this fix!