Fix/#148/duckling invalid dt iso

[ltbringer]

#148

We try to solve the invalid datetimes by allowing our parsers to evaluate entities before returning them.

try:
    time_entity.collect_datetime_values()
except ValueError:
    return None

This within an entity ensures that if we can parse the values only then the entity would be produced. In response to this, we have also changed the type signature of methods.

[codecov-commenter]

Codecov Report

Merging #149 (eae1904) into master (b57a9e5) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #149   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           48        48           
  Lines         2155      2161    +6     
=========================================
+ Hits          2155      2161    +6     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
dialogy/types/entity/base_entity/__init__.py	100.00% <ø> (ø)
dialogy/types/entity/deserialize/__init__.py	100.00% <ø> (ø)
dialogy/plugins/text/duckling_plugin/__init__.py	100.00% <100.00%> (ø)
dialogy/types/entity/time/__init__.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b57a9e5...eae1904. Read the comment docs.