AP_Logger: add new unit `t` for torque in N.m

libraries/AP_Logger/LogStructure.h

@@ -67,6 +67,7 @@ const struct UnitStructure log_Units[] = {
     { '%', "%" },             // percent
     { 'S', "satellites" },    // number of satellites
     { 's', "s" },             // seconds
+    { 't', "N.m" },           // Newton meters, torque
     { 'q', "rpm" },           // rounds per minute. Not SI, but sometimes more intuitive than Hertz
     { 'r', "rad" },           // radians
     { 'U', "deglongitude" },  // degrees of longitude

libraries/AP_Logger/README.md

@@ -68,6 +68,7 @@ Please keep the names consistent with Tools/autotest/param_metadata/param.py:33
 | 's' | "s" | seconds|
 | 'q' | "rpm" | revolutions per minute|  Not an SI unit, but sometimes more intuitive than Hertz|
 | 'r' | "rad" | radians|
+| 't' | "N.m" | Newton meters | torque |
 | 'U' | "deglongitude" | degrees of longitude|
 | 'u' | "ppm" | pulses per minute|
 | 'v' | "V" | Volt|