Move dataloader from dataset

[peterdudfield]

Detailed Description

Move over data loader object from dataset

Context

• Good to divide up dataset a bit
• might have to do it after https://github.com/openclimatefix/nowcasting_dataset/pull/229

Possible Implementation

copy over

• BatchML and its models
• Dataloader

[jacobbieker]

Should this be in nowcasting-dataloader?

[peterdudfield]

oh yea sorry