Add Cost Distribution Grouping class

[MattBuckley-TfN]

Create a new class which manages multiple CostDistribution and provides easy saving, loading and access to them.

• Store CostDistribution for different segments and provide get method for providing specific one
• Load and save from / to:
  • Single stacked CSV
  • Multiple separate cost distribution CSVs
• Produce cost distributions from multiple matrices
• This should be used by caf.distribute's multi-area gravity model

https://github.com/Transport-for-the-North/caf.toolkit/blob/6166e2102aada2c0f2bb43b5630b228e70da19ad/src/caf/toolkit/cost_utils.py#L24