Extract code to procedure `read_grid_inst_eddies`

Extract code from procedure `input_ssh` to new procedure
`read_grid_inst_eddies`. Preparing to move procedure `get_var` to its
own module. We could not move `get_var` before because `get_var` uses
nlon and that would be a circular dependency. With the code extracted
from `input_ssh`, we also move to module `read_grid_inst_eddies_m` the
variables `uniform_lon_lat`, nlon, nlat, step, `corner_whole`,
longitude, latitude, `lon_steps` and `lat_steps`. A new namelist is
read: `read_grid_inst_eddies_nml`. Variable `uniform_lon_lat` moves
from namelist `input_ssh_nml` to namelist `read_grid_inst_eddies_nml`.