VisMF::Read sometimes fails to read output of VisMF::Write

[maikel]

I put this as a reminder here, as I will come back to this with a pull request. I found the reason to why the it is sometimes not possible for me to use amrex::VisMF::Read for files created by amrex::VisMF::Write. At least for Clang and AppleClang it happens when one writes value of type which are in a subrange, like 1E-320 which will fail to be read by iostreams (writing them is ok).

This stackoverflow question is related https://stackoverflow.com/questions/52410931/why-does-clang-stdostream-write-a-double-that-stdistream-cant-read

This should also affect my experience of the VisIt plugin under MacOs