Dynare++ standalone: fix bug when parsing approximation order

It was not possible to set a 2-digit order (i.e. ≥ 10).

Not sure that it was a real problem in practice, given the computational limits.
7 jobs for master in 99 minutes and 59 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #3934
build_binaries

00:10:18

passed #3935
build_doc

00:01:48

 
  Test
failed #3939
test_dynare++

00:02:30

passed #3936
test_matlab

01:29:40

manual #3938
allowed to fail manual
test_octave
manual #3937
allowed to fail manual
test_old_matlab
 
  Deploy
skipped #3940
secure
deploy_manual_unstable
 
Name Stage Failure
failed
test_dynare++ Test
make[2]: *** [check-am] Error 2
make[2]: Leaving directory '/home/gitlab-runner/builds/a354a37e/0/Dynare/dynare/dynare++/tl/testing'
Makefile:365: recipe for target 'check-recursive' failed
make[1]: *** [check-recursive] Error 1
make[1]: Leaving directory '/home/gitlab-runner/builds/a354a37e/0/Dynare/dynare/dynare++/tl'
Makefile:364: recipe for target 'check-recursive' failed
make: *** [check-recursive] Error 1
make: Leaving directory '/home/gitlab-runner/builds/a354a37e/0/Dynare/dynare/dynare++'
ERROR: Job failed: exit status 1