Allow reading files with the duplicated feature names across modalities

[gtca]

See #10 to reproduce the steps:

LoadData(ds = "bmcite") %>% WriteH5MU("bmcite.h5mu"); ReadH5MU("bmcite.h5mu")

This line has to account for duplicated feature names in the multimodal .var data frame.

[gtca]

Resolved by f3d7f1f.