Add methods to coerce to Textual

[ablaom]

Until now, there have been no methods for coercing arrays into the Textual scientific element type, only a fallback that throws an (informative) error. This PR adds a coerce method that applies string to all non-missing elements. In particular, it resolves #193 .

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.42%. Comparing base (7a8c218) to head (8249735).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #218      +/-   ##
==========================================
- Coverage   93.57%   93.42%   -0.16%     
==========================================
  Files           6        6              
  Lines         436      441       +5     
==========================================
+ Hits          408      412       +4     
- Misses         28       29       +1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.