Adding support for GDAL float32 types

I came across a few file formats that could not be loaded giving the error unknown type Float32 from gdal. Fixing that made it write out alot of range convert warnings which also have been fixed in this commit.

Now one can do:
```
	cv::Mat dem = cv::imread(path, cv::IMREAD_LOAD_GDAL | cv::IMREAD_ANYDEPTH);
	std::cout << dem.rows << " " << dem.cols << " " << dem.type();
```
on file types like asc (common dem format as a Ascii text file)