Enhance user experience by changing the defaults

[jinbow]

Hey team, two suggestions:

1. Add examples for this command: > podaac-data-subscriber -h, say using the following as an example:

podaac-data-subscriber -c JASON_CS_S6A_L2_ALT_LR_STD_OST_NRT_F -d ./data/JASON_CS_S6A_L2_ALT_LR_STD_OST_NRT_F --start-date 2020-11-30T00:00:00Z --end-date 2022-11-30T00:00:00Z

2. Anybody who runs > podaac-data-subscriber and Enter does not likely know how to use the tool, so, make > podaac-data-subscriber equivalent to > podaac-data-subscriber -h.

cheers