Support level / time ranges with (ip1,ip2,ip3) triplets.

[neishm]

Currently, only single ip1 values are decoded by fstd2nc.mixins.decode_ip1. This should be extended to properly decode the triplets rp1, rp2, rp3 and handle possible ranges encoded in those elements. Probably this logic should be moved outside of vcoords and put in another mixin, since both forecasts and vertical levels can encode this range information.

This is needed in order to support certain accumulated fields like precipitation.

[neishm]

I originally thought it would be a straightforward case of passing ip1,ip2,ip3 through the rpnpy function DecodeIp, but after some testing it appears that function doesn't always give the appropriate ranges? May need to settle for patching in support piecemeal for each case based on what end users are expecting the ranges to be.

[neishm]

Some support added with 'accum' mixin, for specific case where ip2 represents either the accumulation period (in hours) or the starting hour.