Added support for battery voltage and differential pressure to logging and plot script

apps/systemlib/conversions.c

@@ -42,6 +42,10 @@
 
 #include "conversions.h"
 
+#define air_gas_constant 8.31432f
+#define air_density_sea_level 1.225f
+#define absolute_null_kelvin 273.15f
+
 int16_t
 int16_t_from_bytes(uint8_t bytes[])
 {
@@ -55,3 +59,8 @@ int16_t_from_bytes(uint8_t bytes[])
 
 	return u.w;
 }
+ 
+float get_air_density(float static_pressure, float temperature_celsius)
+{
+	return static_pressure/(air_gas_constant * (temperature_celsius + absolute_null_kelvin));
+}