NetCDF memory leak workaround

[electricsam]

@castelao This is a draft pull request that is built upon PR#66 that I am submitting for review if there is anything that you feel will be useful in CoTeDe. Don't feel obligated to accept it. This is a workaround for https://github.com/Unidata/netcdf4-python/issues/1021 / https://github.com/Unidata/netcdf-c/issues/2626. Theses changes allow for oceandb instances to be passed in, preventing them from being opened and closed repeatedly, which eventually exhausts all the available memory.