Deepreg: Bug: Clash Between Functions For Generating Indices

[NMontanaBrown]

Subject of the issue

Whilst working on #525 #54 I was going through the DataLoaders. The base clase, DataLoader takes specific arguments such as "all", "sample" or None as args, but doesn't take "first":

base DataLoader class that doesn't take "first" as one of the arguments

which is one of the args that is passed to the util:

[which generates the indices](https://github.com/DeepRegNet/DeepReg/blob/db76a33cb3e41a96594626ccf6e3900d3cfa720d/deepreg/dataset/util.py#L62(

[NMontanaBrown]

Not covered in the DataLoader test

[NMontanaBrown]

• Not covered in unpaired loader via a ctrl-f of "first"
• Not covered in paired loader via a ctrl-f of "first"
• Not covered in nifti loader cia a ctrl0f of "sample_label"
• Not covered in h5 loader
• Not covered in grouped loader
• Not covered in dataset load test

[NMontanaBrown]

Pytest fails any tests where "first" are passed to sample_label.

E AssertionError: sample_label must be sample or all or None, got first

[mathpluscode]

@NMontanaBrown let's remove first? it's an old option linked back to some strange dataset. It was back then I want to only evaluate on the first label. But now we always evaluate on all labels and the results per label will be summarized.

[NMontanaBrown]

Ok, undoing.

[mathpluscode]

Ok, undoing.

Still worth a PR to remove first from all existing place ^^. thx to spot this out!