Handle string casting to float for global bounds

Although the ACDD specifies geospatial_(lat,lon)_(min,max) should be
decimal, (See https://geo-ide.noaa.gov/wiki/index.php?title=NetCDF_Attribute_Convention_for_Dataset_Discovery)
occasionally, there are strings for the geospatial lat/lon
min/max.  Handle these and attempt to cast to string if needing to
compute a bounding box from global attributes.