Store domainspace in TimesOperator

[jishnub]

This makes the domainspace of a TimesOperator type-inferred

julia> @inferred domainspace(Derivative() * Multiplication(Fun(), Chebyshev()))
Chebyshev()

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 27.77%. Comparing base (28eb4ea) to head (d1fcd94).
Report is 47 commits behind head on master.

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HEAD has 1 upload less than BASE

Flag	BASE (28eb4ea)	HEAD (d1fcd94)
	9	8

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #440       +/-   ##
===========================================
- Coverage   75.24%   27.77%   -47.47%     
===========================================
  Files          80       80               
  Lines        8421     8406       -15     
===========================================
- Hits         6336     2335     -4001     
- Misses       2085     6071     +3986     

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