Improve splitref: breakdown into 3 functions

[coraliewilliams]

Breakdown splitref into three functions:

1. splitref_two
Split references amongst two collaborators (allowing different proportions for splitting).

• change arguments
• try another way of assigning props

2. splitref_k
Split references amongst multiple collaborators (allowing for only an equal proportion of splits).

3. spiltref_kprop (to do later on)
Split references amongst multiple collaborators (allowing different proportions for splitting).

[coraliewilliams]

Make note to check time efficiency (for cases of large reference lists).