Investigate what is the most apropriate type for `sample_shape` and `target_shape` in ShardDirectorClient

[dmitryagapov]

Now we are using ['100-200','300-400'] to describe height and width. Maybe there will be better alternative ((100, 200), (300,400)), or even create special type for it. https://github.com/intel/openfl/pull/151/files#r692170647

[igor-davidyuk]

Different data science problems may require samples of different dimensionality. If we want to develop a structure to replace strings it should be flexible enough to support different datasets and the logic behind this structure should be well documented.