Fix float-conversion warning in istream operator>>

[martinbudden]

Fixes float-conversion warning that occurs when compiled with -Wfloat-conversion

[greiman]

Thanks for pointing out the warning. Few people use the iostream code that I did many years ago when there were only AVR boards.

I never use pull requests in the release version of SdFat since they often they cause problems with other boards.

I test with a number of boards and static analyzers like Cppcheck and cpplint. These analyzers don't accept C-style castes and I found another warning.

d:\Bill\ArduinoSdFat\libraries\SdFat\src\iostream\ostream.cpp:78:7: warning: 'v' may be used uninitialized in this function [-Wmaybe-uninitialized]
n = -n;
^
d:\Bill\ArduinoSdFat\libraries\SdFat\src/iostream/istream.h:192:12: note: 'v' was declared here
double v;
^

So please try this mod:

  istream& operator>>(float& arg) {
    double v = 0;
    getDouble(&v);
    arg = static_cast<float>(v);
    return *this;
  }

I will put the mod in SdFat-beta and it will finally be in the release version.