[CLEANUP] - Fix issue with NPE when opening ModPartitionerDialog

This issue was introdused with changed
BaseStepDialog.setShellImage(BaseStepDialog.java:1388) algorytm. It
recieves StepMetaInterface in signature but does not pass it to private
mehtod. Private method assumes that BaseStepDialog has protected field
stepMeta.

Unfortatly, if child class (ModPartitionerDialog) also have stepMeta field
as a private field this will overloads parent class value. So this private
field must be removed.
[CLEANUP] - fix junit test avoid using obsolete asserts
PDI-12140 - junit tests provided

junit for different steps combination (regular step, step running in
multiple copies, partitioned steps)
PDI-12140 - IndexOutOfBoundsException when transitioning between different partition schemas

Fix transformation and BaseStep prepare execution logic.
For partitioning we had a case called 'Swim-lanes'. This is a case when we
have 2 steps partitioned with same partitioning schema. For swim-lines we
used to have N_to_N relationship. This is a special mapping when we used
to avoid extra buffers.
The issue was that when we use 2 steps with same patritioning count - but
different partitioning we used to treat this case as a swim lines by
mistake.
This was lead to IOB exception. This case could be workarounded using one
not partitioned step between.
But since we allow to put 2 differenly patritioned steps in line - we
should correctly handle this case. Even it is not optimal transformation
order lead to excessive rowsets.

This commit brings refactoring for steps rowsets relation discovery and
should fix IOOB exception.
Merge pull request #711 from prakapenka/PDI-12140

PDI-12140 - IndexOutOfBoundsException when transitioning between different partition schemas