ISO-TECH IDM103N‎ multimeter support

Thanks to Aurelien Jacobs libsigrok now supports a new multimeter, the ISO-TECH IDM103N. This is a 4000 counts DMM with RS232 connectivity.

The respective patch for enabling this DMM also adds a generic es519xx.c parser (based on the old es51922.c which will be obsoleted soon), which should be able to handle various Cyrustek ES519xx DMM ICs. That'll make it easy to add more DMMs using one of these Cyrustek chips later.

Another new thing in libsigrok is the SR_UNIT_REVOLUTIONS_PER_MINUTE (RPM) unit, which allows us to properly support the DMM's RPM feature.