ybrahim and Moria import: fix hfb merging issue

[luitjansl]

when importing the Ybrahim model, and then calling "write" on the resulting simulation-object, we get an error In the merging of the hfb's - including if we turn of write validation. The error occurs when calling merge_hfb_packages(mf6_hfb_ls, idomain, top, bottom, k)
An error on on calling this function also occurs when importing the Moria model.