implemented parsing of dates with day of year
Merge branch 'master' of https://github.com/yamcs/yamcs
Add support for sign-magnitude integers

The spreadsheet loader defaults to 2’s complement. With this commit,
the representation can be specified between parenthesis in the raw value
column, i.e.: int(2c), int(si). Just ‘int’ will still default to 2’s.

Bug: 2259