compiler: Enable generation of templated function declarations

[FabioLuporini]

No description provided.

[mloubout]

Looks fine, can we add a small test for it?

[codecov]

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 ⚠️
devito/ir/iet/efunc.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            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