Proper warning when converting between types. about bigstatsr HOT 2 CLOSED

• warn when there is a loss of precision (be aware of NA int -> double)
• conserve matrix structure of vector if applicable
• special for unsigned short?
• dispatch for double (no convert)? and others -> need benchmarks

from bigstatsr.

• don't pass on the vectors if no warning?
• always return the same type (checking is not creating a new vector)?
• check as argument of [<-?

from bigstatsr.