api: Minor fixes to arithmetic operations with scalar and tensors

[mloubout]

On top of #2275

[codecov[bot]]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (1bf0991) 86.83% compared to head (582c39c) 77.48%.

Files	Patch %	Lines
devito/types/equation.py	40.00%	3 Missing :warning:
devito/types/basic.py	71.42%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2276 +/- ## ========================================== - Coverage 86.83% 77.48% -9.36% ========================================== Files 229 222 -7 Lines 42435 41853 -582 Branches 7868 7831 -37 ========================================== - Hits 36850 32430 -4420 - Misses 4926 8717 +3791 - Partials 659 706 +47 ```

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