Add support for more expressive walltime

[MarcCote]

Maybe we could change DD:HH:MM:SS to [[[DD:]HH:]MM:]SS in the help message. We could think also about a way to make it more user friendly by allowing one to specify the time unit: -t 2d, -t 23h, -t 120m, -t 3600s

[mgermain]

I like the -t 2d but the only reason I hesitate is because we would work differently than a normal PBS file.

[MarcCote]

We would support both ways.