Serialize Monitor_Pvt data with Protocol Buffers

docs/protobuf/monitor_pvt.proto

@@ -0,0 +1,40 @@
+syntax = "proto3";
+
+package gnss_sdr;
+
+message MonitorPvt {
+
+uint32 tow_at_current_symbol_ms = 1;
+uint32 week = 2;
+double rx_time = 3;
+double user_clk_offset = 4;
+
+double pos_x = 5;
+double pos_y = 6;
+double pos_z = 7;
+double vel_x = 8;
+double vel_y = 9;
+double vel_z = 10;
+
+double cov_xx = 11;
+double cov_yy = 12;
+double cov_zz = 13;
+double cov_xy = 14;
+double cov_yz = 15;
+double cov_zx = 16;
+
+double latitude = 17;
+double longitude = 18;
+double height = 19;
+
+uint32 valid_sats = 20;
+uint32 solution_status = 21;
+uint32 solution_type = 22;
+float ar_ratio_factor = 23;
+float ar_ratio_threshold = 24;
+
+double gdop = 25;
+double pdop = 26;
+double hdop = 27;
+double vdop = 28;
+}