Allow variable number of inputs

[baxpr]

Processor yaml v3 requires the exact number of inputs to be specified. We have a few cases now where some studies may have 1, 2, 3,... of a particular scan (like a T1). For these cases it would be very handy if a spider could run on all of them together, regardless of how many there are - for instance, a registration and averaging pipeline. In other words, allow variable number of a particular input type in the yaml.

[baxpr]

Right now I am doing this by creating multiple yamls: for 1 input, for 2 inputs, for 3 inputs, etc and building each session individually/manually with the correct yaml for the session depending on the number of inputs that are present.