Add function to convert from oncell to ongrid

In relation to issue #144, one solution is to provide the function `Var.oncell_to_ongrid(var)`. The function will check the longitude and potentially the latitude dimension. If the dimensions don't span the entire globe, then it will return an error since the function is undefined. If the var is already ongrid, then the function will return var. Otherwise, we will change the gridding to ongrid. For example, if the longitudes are `[0.5,1.5,...,359.5]`, then the new longitudes will be `[0,1,..,360]`. This can be accomplished by adding an extra longitude point and resampling to the new longitudes. For example, after adding the extra longitude point, the longitudes will be `[0.5, 1.5,..., 359.5, 360..5]` before sampling and the data will be appended to the data belonging to longitude dimension at 0.5.