fix(exif): Check the instance when getting exif tag as float
fix(exif): Check the instance when getting exif tag as float

When a focal length is saved as exifread.utils.Ratio, it can't be got as float.
See https://github.com/ianare/exif-py/blob/develop/exifread/utils.py

sunbingfengPI/OpenSFM_Test,oscarlorentzon/OpenSfM,BrookRoberts/OpenSfM,oscarlorentzon/OpenSfM,sunbingfengPI/OpenSFM_Test,mapillary/OpenSfM,mapillary/OpenSfM,oscarlorentzon/OpenSfM,mapillary/OpenSfM,sunbingfengPI/OpenSFM_Test,BrookRoberts/OpenSfM,oscarlorentzon/OpenSfM,BrookRoberts/OpenSfM,sunbingfengPI/OpenSFM_Test,BrookRoberts/OpenSfM,mapillary/OpenSfM,BrookRoberts/OpenSfM,mapillary/OpenSfM,oscarlorentzon/OpenSfM,sunbingfengPI/OpenSFM_Test