Remove deprecated functions from las.py

[dcslagel]

Description:

This pull-request removes so older and deprecated functions from las.py

Deprecated 3 years ago:

• add_curve_raw()
• add_curve()

Deprecated 2 years ago:

• to_json_old()

Tests updated:

• tests/test_sectionitems.py::test_add_curve_duplicate renamed: tests/test_sectionitems.py::test_append_curve_duplicate
• tests/test_write.py::test_step_unchanged_by_write_2

Test Results

---------- coverage: platform darwin, python 3.8.9-final-0 -----------
Name                       Stmts   Miss  Cover
----------------------------------------------
lasio/__init__.py             28      6    79%
lasio/convert_version.py      20     20     0%
lasio/defaults.py             11      0   100%
lasio/examples.py             42     10    76%
lasio/excel.py                88     34    61%
lasio/exceptions.py            6      0   100%
lasio/las.py                 491     64    87%
lasio/las_items.py           220     30    86%
lasio/las_version.py          50     14    72%
lasio/reader.py              463     27    94%
lasio/writer.py              200      9    96%
----------------------------------------------
TOTAL                       1619    214    87%

-- Let me know if this change could be accepted (or rejected) or needs some additional changes to be approved and merged.

Thank you, DC