Improve create_template functionality

[svilupp]

• Added convenience function create_template to create templates on the fly without having to deal with PT.UserMessage etc. If you specify the keyword argument load_as = "MyName", the template will be immediately loaded to the template registry. See ?create_template for more information and examples.

[codecov]

Codecov Report

Attention: Patch coverage is 93.93939% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 92.43%. Comparing base (4153518) to head (7a04fa4).

Files	Patch %	Lines
src/templates.jl	93.93%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #85      +/-   ##
==========================================
- Coverage   92.43%   92.43%   -0.01%     
==========================================
  Files          38       38              
  Lines        2182     2195      +13     
==========================================
+ Hits         2017     2029      +12     
- Misses        165      166       +1     

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