Conversions: add coordinates and altitude conversion functions
Template: add convenience function to check if coordinates are set
Services: added basic CWOP templates to build APRS packets

Following the standard http://is.gd/APRSProtocol
CWOP Template: use hundreaths of inches for all the rain data
Conversions: add comments
Conversions: add utility function to limit the length of a decimal integer
CWOP template: use max_dec_length instead of modulus operations
CWOP Template: pressure must be in tenth of millibar / hPa