Refactor output

[keurfonluu]

• Changed: dedicated output classes (toughio.ElementOutput and toughio.ConnectionOutput) instead of mere namedtuple toughio.Output.
• Added: option time_steps to read a specific time step to avoid having to read the whole output file when working on a specific time step (so faster).
• Fixed: standard output reader for outputs generated by early versions of TOUGH2.

Reminders:

• [x] Run invoke format to make sure the code follows the style guide,
• [x] Add tests for new features or tests that would have caught the bug that you're fixing,
• [x] Write detailed docstrings for all functions, classes and/or methods,
• [x] If adding new functionality, unit test it and add it to the documentation.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.09272% with 42 lines in your changes are missing coverage. Please review.

Project coverage is 90.20%. Comparing base (4894d6d) to head (636014e). Report is 16 commits behind head on master.

Files	Patch %	Lines
toughio/_io/output/_common.py	73.33%	24 Missing :warning:
toughio/_io/output/tough/_tough.py	90.38%	5 Missing :warning:
toughio/_io/output/_helpers.py	80.00%	4 Missing :warning:
toughio/_io/output/petrasim/_petrasim.py	92.10%	3 Missing :warning:
toughio/_cli/_export.py	60.00%	2 Missing :warning:
toughio/_io/output/csv/_csv.py	95.34%	2 Missing :warning:
toughio/_mesh/_mesh.py	71.42%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #170 +/- ## ========================================== - Coverage 90.21% 90.20% -0.01% ========================================== Files 97 97 Lines 6283 6433 +150 ========================================== + Hits 5668 5803 +135 - Misses 615 630 +15 ```

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