discretize a BilinearForm with spaces of different degree

[FrederikSchnack]

Closed and moved to #517 because of the wrong base branch.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.83%	✅ 77.60%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (42b214e)	30762	18640	60.59%
Head commit (8f3c696)	61042 (+30280)	37496 (+18856)	61.43% (+0.83%)

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 (#516)	741	575	77.60%

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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