refactor read/write to blocked array; extern templates

- read/write operations in BA::Array have been refactored to
  use a common iterator which computes the needed Roi's
- optionally, a library can be built with the most commond
  data types (BUILD_COMMON_DTYPE_LIBRARY cmake option).
  However, this does not reduce compile times. To be
  investigated.
- Implement applyRelabeling() function
- some cleanup