Changing unit description to units for consistency

[hellovolcano]

• This contribution adheres to CONTRIBUTING.md.

What does this Pull Request accomplish?

Updates the instances of unit_description to units for consistency.

Why should this Pull Request be merged?

Fixes #168

What testing has been done?

Unit tests pass

[github-actions]

Test Results

    44 files  ±0      44 suites  ±0   1h 29m 51s ⏱️ -22s
 2 197 tests ±0   2 197 ✅ ±0  0 💤 ±0  0 ❌ ±0 
49 875 runs  ±0  49 875 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 1532be2. ± Comparison against base commit 764a576.

This pull request removes 4 and adds 4 tests. Note that renamed tests count towards both.

tests.unit.waveform.test_analog_waveform ‑ test___invalid_type___set_unit_description___raises_type_error
tests.unit.waveform.test_analog_waveform ‑ test___waveform___set_unit_description___sets_extended_property
tests.unit.waveform.test_spectrum ‑ test___invalid_type___set_unit_description___raises_type_error
tests.unit.waveform.test_spectrum ‑ test___spectrum___set_unit_description___sets_extended_property

tests.unit.waveform.test_analog_waveform ‑ test___invalid_type___set_units___raises_type_error
tests.unit.waveform.test_analog_waveform ‑ test___waveform___set_units___sets_extended_property
tests.unit.waveform.test_spectrum ‑ test___invalid_type___set_units___raises_type_error
tests.unit.waveform.test_spectrum ‑ test___spectrum___set_units___sets_extended_property