ENH: Avoiding conversion to NumPy Scalar. Implementation Copy-paste tag in conv_template.py

This pr is based on same rationale and improvement suggested in @raulcota 's pr #2941. For some types, it is a lot faster to just extract the value directly from the Python scalar. It take care of main concerned of code duplication. 

Implementation of copy and paste tag in <code>cove_template.py</code>

Example
-------------
Here is demo template
<pre>
/**begin copy 
 *@hi@
 */
Hello! This is just an dummy line
 /**end copy**/

First I will write some useless line, then include code pasted to variable 
<b>hi</b> as /**paste @hi@**/
</pre>

Output:
<pre>
First I will write some useless line, then include code pasted to variable 
<b>hi</b> as Hello! This is just an dummy line
</pre>

Usage
---------
Here, we could avoid duplicating the whole function body just to add  one specific check as per dtype.