Default number of factors in APEX factor analysis

[hsun3163]

Based on Figure 2A of this preprint:

A versatile toolkit for molecular QTL mapping and meta-analysis at scale
Corbin Quick, Li Guan, Zilin Li, Xihao Li, Rounak Dey, Yaowu Liu, Laura Scott, Xihong Lin
bioRxiv 2020.12.18.423490; doi: https://doi.org/10.1101/2020.12.18.423490

The performance gained by increasing more common factors seems to stop at ~30 for all three dataset. And by default N cannot be zero. So the default of N is set to be 30

Noted, setting N = 30 will force the output to have 30 factors.

[hsun3163]

Due to #162, a run with bicv factor was initiated to avoid factor analysis being a blocker. In this initiated analysis. The factor is set to the same number of maximum factor set forth in the peer r wrapper.

Setting 60 will provides a output with 60 factors and was finished in less than 1 hr.