Reorganise namespaces

[mcocdawc]

Created explicit src/chemcoord/xyz_functions.py and src/chemcoord/zmat_functions.py files that pull in all the names for the public namespace.

Advantages:

• self-explaining structure of what is exported to the public namespace
• get rid of @export decorator
• get rid of the shenanigan of modifying sys.modules

Fixes #110

minor fix on the way:
removed the UTF8 header declaration

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 50 lines in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (521240f) to head (0b7bc54).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/chemcoord/__init__.py	0.00%	12 Missing ⚠️
...hemcoord/_internal_coordinates/_zmat_class_core.py	0.00%	7 Missing ⚠️
...cartesian_coordinates/_cartesian_class_get_zmat.py	0.00%	5 Missing ⚠️
...rd/_cartesian_coordinates/_cartesian_class_core.py	0.00%	4 Missing ⚠️
...cartesian_coordinates/_cartesian_class_symmetry.py	0.00%	3 Missing ⚠️
...ord/_cartesian_coordinates/cartesian_class_main.py	0.00%	3 Missing ⚠️
.../chemcoord/_cartesian_coordinates/xyz_functions.py	0.00%	2 Missing ⚠️
...oord/_internal_coordinates/_zmat_transformation.py	0.00%	2 Missing ⚠️
.../chemcoord/_internal_coordinates/zmat_functions.py	0.00%	2 Missing ⚠️
...ord/_cartesian_coordinates/_cart_transformation.py	0.00%	1 Missing ⚠️
... and 9 more

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #111   +/-   ##
======================================
  Coverage    0.00%   0.00%           
======================================
  Files          27      29    +2     
  Lines        2787    2773   -14     
======================================
+ Misses       2787    2773   -14     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.