Implementation of the BDY package

[ctlod]

Summary of code changes
================

1. Add BDY module, including bdytides.F90 for tidal harmonic
   forcing. BDY works for key_dynspg_exp, key_dynspg_ts and
   key_dynspg_flt. FRS and Flather schemes are in separate
   routines. Note BDY does not yet include ice fields.

2. Add calls to BDY routines in:
       step.F90
       dynpsg_flt.F90
       dynspg_ts.F90
       dynnxt.F90
       tranxt.F90

3. Change iom_gettime (and iom_ioipsl_gettime, iom_nf90_gettime) so we
   can choose to read the record dimension as time and to read extra
   attributes of the time coordinate. "cdvar" is now an optional argument,
   so order of arguments is changed, so call from obcdta.F90 is
   changed.

4. Bug fix for VVL code in dynnxt.F90. Velocities must be unweighted
   by layer thickness *before* open boundary conditions applied.

5. Alternative option for choice of layer thicknesses in
   domvvl.F90. If key_sigma_vvl set then distribute SSH anomaly
   equally over all layers. Prevents the possibility of negative layer
   thicknesses in shallow water.