Cast field reads also in BC

Using mixed precision, field reads were not isolated in the boundary kernels. This leads to calculations in lower precision in the BC

added feature proposal label

requested review from @ed94aqyc

assigned to @holzer

added 3 commits

• fdeec94f...2068f36f - 2 commits from branch pycodegen:master
• 7af78f3c - Merge branch 'master' of i10git.cs.fau.de:pycodegen/lbmpy into CastFieldReads

Compare with previous version

requested review from @da15siwa and removed review request for @ed94aqyc

requested review from @ed94aqyc and removed review request for @da15siwa

mentioned in commit d193fd9d

merged