Refactor: Change for all bounday conditions the usage of field.data to field

With the new changes, it may make more sense to give the boundary conditions also Fields as parameters and not their data array. (With regard to the changes in #124 )

Start by reading the boundary-condition code and issue #124, then locate every use of field.data in boundary-condition handling. Determine how those parameters currently receive data arrays and update the design so all boundary conditions use Fields instead; the work is done when all such usages are converted consistently and the existing simulation checks still pass.

Written by the indexing model from the issue text.