doc: correct 'Pstar' units (#325)

* doc: correct 'Pstar' units

`Pstar` is really in N/m^2, but is incorrectly listed as being in N/m.

Fix that.

* .github: add '.md' extension to PULL_REQUEST_TEMPLATE

The GitHub documentation implicitely suggests using a '.md' extension
for pull request template [1].

Let's follow that recommendation.

[1] https://help.github.com/en/github/building-a-strong-community/manually-creating-a-single-issue-template-for-your-repository

doc/source/icepack_index.rst

@@ -352,7 +352,7 @@ either Celsius or Kelvin units).
    "PP_net", "total primary productivity per grid cell", "mg C/m\ :math:`^2`/s"
    "precip_units", ":math:`\bullet` liquid precipitation data units", ""
    "print_points", ":math:`\bullet` if true, print point data", "F"
-   "Pstar", "ice strength parameter", "2.75\ :math:`\times`\ 10\ :math:`^4`\ N/m"
+   "Pstar", "ice strength parameter", "2.75\ :math:`\times`\ 10\ :math:`^4`\ N/m\ :math:`^2`"
    "puny", "a small positive number", "1\ :math:`\times`\ 10\ :math:`^{-11}`" 
    "**Q**", "", ""
    "Qa", "specific humidity at 10 m", "kg/kg"

doc/source/science_guide/sg_mechanical.rst

@@ -252,7 +252,7 @@ namelist parameter ``kstrength`` = 0, we use the strength formula from
    P = P^* h \exp[-C(1-a_i)],
    :label: hib-strength
 
-where :math:`P^* = 27,500 \, \mathrm {N/m}` and :math:`C = 20` are
+where :math:`P^* = 27,500 \, \mathrm {N/m^2}` and :math:`C = 20` are
 empirical constants, and :math:`h` is the mean ice thickness.
 Alternatively, setting ``kstrength`` = 1 gives an ice strength closely
 related to the ridging scheme. Following