ice_history: correct units for 'sigP'

The intenal ice pressure 'sigP' is is units of N/m, as can be seen in
ice_dyn_shared::principal_stress. However, the corresponding history
variable is wrongly defined in ice_history::init_hist with unit '1'
(dimensionless). This means the wrong unit is written to the NetCDF
history output. This dates back to the introduction of that variable in
6ed2359 (Added pressure, modified norm of principal stresses and made
small modifs to basal stress following Till's comments, 2018-03-02).

Fix the unit. While at it, add an entry for 'sigP' in the index, from
which this variable is missing.

Reported-by: Frederic Dupont <frederic.dupont@canada.ca>
Reported-by: Jean-Francois Lemieux <jean-francois.lemieux@canada.ca>

cicecore/cicedyn/analysis/ice_history.F90

@@ -1341,7 +1341,7 @@ subroutine init_hist (dt)
              "sig2 is instantaneous" // trim(description), c1, c0, &
              ns1, f_sig2)
 
-         call define_hist_field(n_sigP,"sigP","1",gridstr2d, gridstr, &
+         call define_hist_field(n_sigP,"sigP","N/m",gridstr2d, gridstr, &
              "ice pressure",                       &
              "sigP is instantaneous" // trim(description), c1, c0, &
              ns1, f_sigP)

doc/source/cice_index.rst

@@ -621,6 +621,7 @@ either Celsius or Kelvin units).  Deprecated parameters are listed at the end.
    "shlat", "southern latitude of artificial mask edge", "30\ :math:`^\circ`\ N"
    "shortwave", "flag for shortwave parameterization (‘ccsm3’ or ‘dEdd’)", ""
    "sig1(2)", "principal stress components (diagnostic)", ""
+   "sigP", "internal ice pressure", ""
    "sil", "silicate concentration", "mmol/m\ :math:`^3`"
    "sinw", "sine of the turning angle in water", "0."
    "Sinz", "ice salinity profile", "ppt"