Merge branch...

Merge branch '17-does-not-compile-error-cannot-convert-walberla-real_t-aka-double-to-real_t-aka-float' into 'master'

Resolve "Does not compile: error: cannot convert ‘walberla::real_t* {aka double*}’ to ‘real_t* {aka float*}’"

Closes #17

See merge request !9