Bugfix - datasets now converted to list as intended in compute_ld_coeffs

[MichaelAbdul-Masih]

In bundle.py the function compute_ld_coeffs was missing logic to convert datasets into a list before trying to append an additional dataset. This caused an error pointed out in the following issue: https://github.com/phoebe-project/phoebe2-docs/issues/36

We have added the missing logic and now the error no longer occurs.