Adding rules and methods for hessian of sum of scalar CellFields and product of two scalar CellFields

[kishore-nori]

• added rules and methods for hessian of sum of scalar Fields (hence CellFields)
• added rules and methods for hessian of product of two scalar Fields (hence CellFields)
• added corresponding tests to check the new functionality

this closes issue #875

[codecov]

Codecov Report

Attention: Patch coverage is 35.89744% with 25 lines in your changes missing coverage. Please review.

Project coverage is 88.80%. Comparing base (fd2a682) to head (8d4d28f).

Files with missing lines	Patch %	Lines
src/Fields/ApplyOptimizations.jl	0.00%	22 Missing ⚠️
src/Fields/FieldsInterfaces.jl	82.35%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1053      +/-   ##
==========================================
- Coverage   88.87%   88.80%   -0.07%     
==========================================
  Files         195      195              
  Lines       23575    23610      +35     
==========================================
+ Hits        20953    20968      +15     
- Misses       2622     2642      +20     

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