Support subsetting prior to plotting

[falkamelung]

When a subsetting option is given it should run subset.py prior to plotting. Subsetting options could be:

 -l 30.5 30.8 -L 130.3 130.9
--lat 32.5 33.0 --lon 130.2 130.6 
 --subset-SNWE 25.921:25.958,-80.125:-80.118

This is based on the subset.py options:

  subset.py geo_velocity.h5       -l 30.5 30.8 -L 130.3 130.9
  subset.py velocity.py --lat 32.5 33.0 --lon 130.2 130.6 
 subset.py velocity --subset-SNWE 25.921:25.958,-80.125:-80.118