*ECI* functions: higher accuracy

Enabled much better accuracy for ECI functions by using Vallado and other methods accounting for nutation etc.

Interfaces for eci2ecef() and ecef2eci() are similar to Matlab now. Other functions interfaces are unchanged.

correct ecef2ned: Add missing namespace

Thanks @MartinKoch123

also various lint, CI fixes over time.

deduplicate validations

v3.0.4

doc

add lint test

v3.0.3

add lint test

CI: use function-based tests

improve flexibility and control of tests by using function-based tests instead of script-based

CI: add Matlab Travis-CI

Use the new Focal (Ubuntu 20.04) Travis-CI image with Matlab on Travis-CI

use R2019b argument validation

Help ensure better correctness and error feedback by using Matlab R2019b argument validation

Use datetime() for robustness

Use datetime class for robustness. GNU Octave users can use Tablicious for datetime.

use as package to control scope

Since the function names overlap with Matlab Mapping Toolbox and other user-created functions, using matmap3d as a package helps ensure you're in the desired namespace.
The top-level matmap3d/ must be on Matlab path, and then:

import matmap3d.*

and/or

matlab.geodetic2ecef(..)

bugfix: eci2ecef, ecef2eci non-scalar

Thanks @brethvoice for pointing out that ecef2eci and eci2ecef didn't work for non-scalar inputs.