Add codepath to initialize an empty AlignedTextGrid

[chrisbrickhouse]

See #169

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.33%. Comparing base (8aaca9e) to head (4a927a5).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #170 +/- ## ========================================== + Coverage 91.22% 91.33% +0.11% ========================================== Files 14 14 Lines 1253 1269 +16 ========================================== + Hits 1143 1159 +16 Misses 110 110 ```

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[JoFrhwld]

I like it!

[JoFrhwld]

This is now on released and published to pypi as v0.6.4