Config option that can be set to turn off the semantics phase

[patritzenfeld]

Maybe deferred to a separate PR: a config option that can be set to turn off the semantics phase (whatever that phase is, according to the split-phases setting).

One use-case: configuring a task complete with test suite etc., but turning the semantics stuff off during an exam setting (not only hiding the semantics output, but really preventing its possibly costly computation), but being able to simply turn it on after the fact.

Originally posted by @jvoigtlaender in #39 (comment)

[jvoigtlaender]

When the sample solution is checked as part of the validity checking of a task configuration (see #24 (comment)), the whole semantics phase should be run, regardless of what the new config option here says.