Replace hardwired 0.61_r8 coefficient with a parameter

This number appears often and should be replaced with a parameter.

> `git grep '0.61_r8'`

```
src/biogeophys/BareGroundFluxesMod.F90: dthv = dth(p)*(1._r8+0.61_r8*forc_q(c))+0.61_r8*forc_th(c)*dqh(p)
src/biogeophys/BareGroundFluxesMod.F90: thvstar = tstar*(1._r8+0.61_r8*forc_q(c)) + 0.61_r8*forc_th(c)*qstar
src/biogeophys/BiogeophysPreFluxCalcsMod.F90: thv(c) = forc_th(c)*(1._r8+0.61_r8*forc_q(c))
src/biogeophys/CanopyFluxesMod.F90: dthv(p) = dth(p)*(1._r8+0.61_r8*forc_q(c))+0.61_r8*forc_th(c)*dqh(p)
src/biogeophys/CanopyFluxesMod.F90: thvstar = tstar*(1._r8+0.61_r8*forc_q(c)) + 0.61_r8*forc_th(c)*qstar
src/biogeophys/LakeFluxesMod.F90: thv(c) = forc_th(c)*(1._r8+0.61_r8*forc_q(c)) ! virtual potential T
src/biogeophys/LakeFluxesMod.F90: dthv = dth(p)*(1._r8+0.61_r8*forc_q(c))+0.61_r8*forc_th(c)*dqh(p)
src/biogeophys/LakeFluxesMod.F90: thvstar=tstar*(1._r8+0.61_r8*forc_q(c)) + 0.61_r8*forc_th(c)*qstar
src/biogeophys/SurfaceAlbedoMod.F90: albdry(1:20,2) = (/0.61_r8,0.57_r8,0.53_r8,0.51_r8,0.49_r8,0.48_r8,0.45_r8,0.43_r8,&
src/biogeophys/UrbanFluxesMod.F90: thv_g(l) = forc_th(g)*(1._r8+0.61_r8*forc_q(g))
src/biogeophys/UrbanFluxesMod.F90: dthv = dth(l)*(1._r8+0.61_r8*forc_q(g))+0.61_r8*forc_th(g)*dqh(l)
src/biogeophys/UrbanFluxesMod.F90: thvstar = tstar*(1._r8+0.61_r8*forc_q(g)) + 0.61_r8*forc_th(g)*qstar
```