Allow all modbus slave id's

Normal slave id's range from 0 to 247. But slave id's 248 to 255 are
sometimes used to access administration/testing registers. So this
should also be allowed.
Remove current value from graph name when exporting to file
Fixed data export (very large times)

Scientific notation is not used anymore on very large numbers;
Data export now clears file before writing new data;
Improved poll interval scheduling

Make sure the poll interval is the time between the start of 2 polls and
not the time between end of poll and start of next poll;
Prepare for release v0.4.0