[feat] Add constructor for Earth gravity model that doesn't require defining a directory

[kyle-cochran]

Is your feature request related to a problem? Please describe.
When defining an Earth gravity model, we have a required directory/aDataDirectory argument. This argument is meant to allow defining gravity model files manually. (e.g. the data underlying the EGM2008). If the argument is set to Undefined, OSTk will manage the files automatically.

In 99% of cases, we pass in Directory::Undefined(). Being required to do this every time is a pain (and looks kinda ugly).

Describe the solution you'd like
We should allow this to be optional, and default it to Undefined. Either by converting existing constructor to make the aDataDirectory argument optional, or adding a new constructor without it.

[kyle-cochran]

Solved in #256