*  split into subpackages axi4 and axi4lite
*  moved each module into its subpackage
*  shortened names by removing Axi-prefixes wherever possible
*  moved Configs and IOs into corresponding companion object
*  replaced some printlns/printfs with Logging facilities

* AXI4Lite interface
* flexible ControlRegister class hierarchy: constants, single values,
  virtual registers (callbacks)
* implemented unit test cases
* had to implement Axi4LiteProgrammableMaster for batch testing