API Backward Compatibility and Deprecations

This MR re-introduces a number of legacy APIs to the new backend in order to retain backward-compatibility; some of them are being deprecated. This MR, together with !401 (merged), prepares the migration of lbmpy and walberla to the new backend.

requested review from @holzer

assigned to @da15siwa

added 3 commits

• d988cb89...1e7b4f56 - 2 commits from branch backend-rework
• b22635d5 - Merge branch 'backend-rework' into fhennig/api-compatibility

Compare with previous version

mentioned in commit e295e9f9

merged