cluster_prepare_fastqtl fails based on naming scheme of sample map

[evin-padhi]

Hi,

We've been running the sQTL pipeline recently but have encountered some errors. Ive been able to map it down to line 165 in https://github.com/broadinstitute/gtex-pipeline/blob/master/qtl/leafcutter/src/cluster_prepare_fastqtl.py

The issue occurs when the index of the sample_participant_look up is numeric. When it tries to execute the assertion the types between the index and columns are different so it errors out even though they are the same value.