fix for Problem adding LinearTrend #675

[OnnoEbbens]

Close issue 675 and change the behavior of _check_response_tmax in such a way that this check is only done for stressmodels with a response function. Stressmodels without a response function, such as 'LinearTrend', will not be checked.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.02% (target: +0.00%)	✅ 39.29%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (0d7e816)	5853	4378	74.80%
Head commit (84ca7cc)	5857 (+4)	4382 (+4)	74.82% (+0.02%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#676)	28	11	39.29%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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