Use template and dates as positional arguments

We rewrite `inst_eddies_Aviso.py` so it is more convenient to use. The
script is only useful for several input files. Otherwise we can
directly use `inst_eddies.py`. So instead of optional arguments
template and last date, we use positional arguments. Then there is no
point in distinguishing dirname and basename. Also, it does not make
sense any longer to read the date from the content of the first
file. The script becomes more general: we do not assume that all input
NetCDF files are in the same -directory and that the time coordinate
is in days since 1950-1-1. The interface is also clearer.