Support other TLE I/O formats

[csete]

CelesTrak is now providing TLE data in other, more flexible formats that will also work once the 5-digit NORAD catalog ID runs over. We should add support for importing and exporting satellite data in at least one of these formats.

References:

1. A New Way to Obtain GP Data
2. NORAD General Perturbations (GP) Element Sets Current Data