Bounds variable search assumes time dimension name

[angus-g]

In the following code segment, the search for relevant bounds variables hard-codes the name of the time dimension as time, but this isn't always the case. Perhaps this should use cc.netcdf_utils.find_time_dimension?

https://github.com/COSIMA/cosima-cookbook/blob/ac2d674bf7639558e125d390457fcf6692b92b7a/cosima_cookbook/querying.py#L420-L422