Adds logic to compute dataset bounding box using global attrs
Transform zero area global attribute bounding boxes to points
Fix box coordinate order for d3 maps
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.
Merge pull request #128 from ioos/boundingbox

Adds logic to compute dataset bounding box using global attrs