Revert "Hack to get assembly URLs from EBI instead of NCBI, since API is broken"

This reverts commit 246037bb586246803e77c39d4e492e0b51a03cdb.
Fix tests after NCBI API results changed
Reduce the number of objects asked per request from 250 to 50 for speedup.

NCBI takes too long to return when asked for many objects in one request.
It's faster overall to make many smaller requests sequentially.
0.0.8