i.evapo.mh: fix radiation conversion factor (#5600)

Previously, the radiation input (ra) was multiplied by 86.4, incorrectly boosting the energy input and resulting in overestimated evapotranspiration values.

This PR corrects the unit conversion:

Changes ra = ra * (24.0 * 60.0 * 60.0 / 1000.0); to ra = ra * (24.0 * 60.0 * 60.0 / 1000000.0);
This matches the correct SI conversion where 1 W/m² = 0.0864 MJ/m²/day

Author: jayneel-shah18

imagery/i.evapo.mh/mh_eto.c

@@ -12,7 +12,7 @@ double mh_eto(double ra, double tavg, double tmax, double tmin, double p)
     if (tavg > 100.0) {
         tavg = tavg - 273.15; /*in case temperature is in Kelvin */
     }
-    ra = ra * (24.0 * 60.0 * 60.0 / 1000.0); /*convert W -> MJ/d */
+    ra = ra * (24.0 * 60.0 * 60.0 / 1000000.0); /*convert W -> MJ/d */
     result = 0.0013 * 0.408 * ra * (tavg + 17.0) * pow((td - 0.0123 * p), 0.76);
     return result;
 }

imagery/i.evapo.mh/mh_original.c

@@ -13,7 +13,7 @@ double mh_original(double ra, double tavg, double tmax, double tmin,
     if (tavg > 100.0) {
         tavg = tavg - 273.15; /*in case Temperature is in Kelvin */
     }
-    ra = ra * (24.0 * 60.0 * 60.0 / 1000.0); /*convert W -> MJ/d */
+    ra = ra * (24.0 * 60.0 * 60.0 / 1000000.0); /*convert W -> MJ/d */
     result = 0.0023 * 0.408 * ra * (tavg + 17.8) * pow(td, 0.5);
     return result;
 }

imagery/i.evapo.mh/mh_samani.c

@@ -11,7 +11,7 @@ double mh_samani(double ra, double tavg, double tmax, double tmin)
     if (tavg > 100.0) {
         tavg = tavg - 273.15; /*in case Temperature is in Kelvin */
     }
-    ra = ra * (24.0 * 60.0 * 60.0 / 1000.0); /* convert W -> MJ/d */
+    ra = ra * (24.0 * 60.0 * 60.0 / 1000000.0); /* convert W -> MJ/d */
     result =
         0.0023 * 0.408 * ra * pow(td, 0.5) * ((tmax + tmin) / 2 + 17.8) / 2.45;
     return result;