portfolio: manage real variable as objective variable

## Expected behavior

Choco should be able to work with MPS files with multiple cores (`-p 3` option in choco-parsers).

## Actual behavior

The mps solver returns an error:

c [-p, 3, /home/pchtsp/Documents/resources/choco-parsers/src/test/resources/mps/example1.mps]
c 3 solvers in parallel
c Unimodular: false
c Unimodular: false
c Unimodular: false
Exception in thread "main" java.lang.UnsupportedOperationException: ParallelPortfolio cannot deal with real variable objective optimization problems
at org.chocosolver.solver.ParallelPortfolio.check(ParallelPortfolio.java:412)
at org.chocosolver.solver.ParallelPortfolio.prepare(ParallelPortfolio.java:254)
at org.chocosolver.solver.ParallelPortfolio.solve(ParallelPortfolio.java:188)
at org.chocosolver.parser.mps.MPS.manyThread(MPS.java:181)
at org.chocosolver.parser.mps.MPS.solve(MPS.java:150)
at org.chocosolver.parser.mps.ChocoMPS.main(ChocoMPS.java:27)
s UNKNOWN

## Possible Solution

Check issue in choco-parsers: https://github.com/chocoteam/choco-parsers/issues/15

citing @cprudhom:

> That’s a limitation of the current portfolio, it doesn’t manage real variable as objective variable. That should be modified too. Can you create another issue on that point too (related to choco, not parser) ? Btw, thank you for the MWE.

## Steps to Reproduce (for bugs)

Using one of the mps examples. I even tried converting all the variables into integers in the mps and still got the error.

java -cp .:choco-parsers-4.0.5-SNAPSHOT-with-dependencies.jar org.chocosolver.parser.mps.ChocoMPS -p 3 choco-parsers/src/test/resources/mps/example1.mps

## Context

Trying to use choco with multiple processors to solve problems with the mps interface in choco-parsers.

## Environment

* Choco-solver version: `choco-solver-4.0.7-SNAPSHOT` + `choco-parsers-4.0.5-SNAPSHOT`
* Java version: `openjdk version "1.8.0_162"`
* PC details: I'm using Ubuntu 16.04 LTS 64 bits with an i7 processor and 16GB RAM.