Change node identifiers from Matlab

The node identifiers in Matlab use the date index in the array of
processed dates. This is not simply the difference between the current
date and the first date if there are missing dates. This means that
converting between node index in Matlab and date and eddy indices, one
needs the array of processed dates. Morever, the conversion from date
and eddy indices to node index requires a search through the array of
dates. So we do not keep the Matlab node identifier. We use k and n as
described in the documentation.

For this commit, we need to revert in part commit 68da90cd. We have to
read `date_num` from the Matlab files, and we create function
`adjust_n` to convert from Matlab node identifier to the new node identifier.