Re-organize API exports

[markmc]

Below is a list of out current API exports and a proposal for how they should be exposed

Top-level package:

• sdg.generate_data() - This is the main interface used by the CLI. Long-term I question whether this belongs in the library (see #POC: move generate_data() back from the SDG library instructlab#1519)
• sdg.SDG -> I propose removing it in Consider updating API design for clarity #61
• sdg.pipeline.Pipeline -> sdg.Pipeline (same for PipelineContext)
• sdg.block.Block -> sdg.Block

Blocks:

• sdg.filterblock.FilterByValueBlock -> sdg.FilterByValueBlock
• sdg.importblock.FilterByValueBlock -> sdg.ImportBlock
• sdg.llmblock.LLMBlock -> sdg.LLMBlock
• sdg.llmblock.ConditionalLLMBlock -> sdg.ConditionalLLMBlock
• sdg.utilblocks.SamplePopulatorBlock -> sdg.SamplePopulatorBlock
• sdg.utilblocks.SelectorBlock -> sdg.SelectorBlock
• sdg.utilblocks.CombineColumnsBlock -> sdg.CombineColumnsBlock

Default pipelines:

• sdg.SIMPLE_PIPELINES_PACKAGE and sdg.FULL_PIPELINES_PACKAGE

Exceptions:

• FilterByValueBlockError
• EmptyDatasetError
• PipelineConfigParserError
• GenerateException
• TaxonomyReadingException

[russellb]

related issue: #30