Cysgp4

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New Features
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- Add an option to allow querying the rotation matrices that would convert any
vector from the (moving) satellite frame to the ECI-aligned frame to the
`propagate_many` function. It is noted that the origin of this ECI-aligned
frame is still at the satellite center (See function description.)

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This is a pure maintenance release. No features have been added.

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New Features
------------
- Add an option to choose the satellite coordinate frame in the
`propagate_many` function. The two available choices differ in the order
of the Cartesian axes (how x/y/z are mapped to motion and nadir directions)
and the choice of angles. (See function description.)

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Bugfixes
~~~~~~~~~~
- `PyDateTime` was using wrong time zone if current time was requested (e.g.,
by calling it with empty init arguments, or setting the `datetime` property
to None). The wrong behavior had its origin in a call to `datetime.now()`
while the correct call must be `datetime.utcnow` (as `PyDateTime` is
defined in the UTC frame. [13]

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New Features
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- Add observer coordinates (in eci frame and moving satellite frame) to
propagate_many function. This can be useful if one wants to calculate
satellite communication antenna gains w.r.t. observer on Earth. [12]

Bugfixes
~~~~~~~~~~
- In the `tle_linestrings_from_orbital_parameters` two parameters were missing
(eccentricity and argument of perigee), which are necessary for a complete
orbit description. [11]

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New Features
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- Migrate to the latest `libsgp4` version (master, hash: f5cb54b3), which can
be downloaded from Dan Warners repository: https://github.com/dnwrnr/sgp4.
Thanks to Cees Bassa for pointing out that the ancient version that was
previously in use was having accuracy issues.