compiler: Enable generation of templated function declarations

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.00000% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 86.67%. Comparing base (545425a) to head (b511ffe).

Files	Patch %	Lines
devito/ir/iet/visitors.py	64.28%	3 Missing and 2 partials :warning:
devito/ir/iet/efunc.py	75.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2333 +/- ## ========================================== - Coverage 86.68% 86.67% -0.01% ========================================== Files 232 232 Lines 43413 43423 +10 Branches 8057 8061 +4 ========================================== + Hits 37632 37639 +7 - Misses 5078 5079 +1 - Partials 703 705 +2 ```

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[FabioLuporini]

a test is added is PRO (the test you've seen would fail to compile without the changes here)

but I can maybe add a simple IET test too

[FabioLuporini]

@mloubout tests added