"fitted" output is incorrect

@gaow @zouyuxin The "fitted" output from calling `mvsusie` is clearly not right. See [here](https://github.com/stephenslab/mvsusieR/blob/master/tests/testthat/test_mvsusie_predict.R) for an illustration of this. (Note I didn't check `mvsusie_rss`.) Do you know if this might lead to issues elsewhere?