add slots to DAQmx-property-containing classes

[zhindes]

• This contribution adheres to CONTRIBUTING.md.
• I've updated CHANGELOG.md if applicable.
• I've added tests applicable for this pull request

What does this Pull Request accomplish?

Prevents inadvertent misuse of the DAQmx API by raising an AttributeError if a user attempts to use a DAQmx Property that doesn't exist. Closes #656.

Why should this Pull Request be merged?

This avoids really tricky bugs.

What testing has been done?

• Added tests for all the modified attribute-containing classes.
• Added tests for weakref support on Task and WatchdogTask, since we want to maintain that.
• Ran the rest of existing tests.

[github-actions]

Test Results

0 files   -     40  0 suites   - 40   0s ⏱️ - 1h 2m 33s
0 tests  -  2 350  0 ✅  -  1 996  0 💤  -   354  0 ❌ ±0 
0 runs   - 49 828  0 ✅  - 42 484  0 💤  - 7 344  0 ❌ ±0 

Results for commit 8ebd5f8. ± Comparison against base commit d2e7efc.

♻️ This comment has been updated with latest results.

[bkeryan]

Approved with suggestions