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 64.10256% with 14 lines in your changes missing coverage. Please review.

Project coverage is 89.37%. Comparing base (a8b5028) to head (a7c046a).
Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Fields/ApplyOptimizations.jl	50.00%	11 Missing ⚠️
src/Fields/FieldsInterfaces.jl	82.35%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1053      +/-   ##
==========================================
+ Coverage   89.36%   89.37%   +0.01%     
==========================================
  Files         197      197              
  Lines       23935    23970      +35     
==========================================
+ Hits        21389    21423      +34     
- Misses       2546     2547       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[JordiManyer]

Hey @kishore-nori ! I need this actually, can we merge it? I think we would only need to pull from master and increase a bit the number of tests.

[JordiManyer]

Also closes #69

[kishore-nori]

Hi @JordiManyer I have added few more tests, and it improves the codecov but seem like haven't been able to trigger the code additions in src/Fields/ApplyOptimizations.jl. Having said that, it works on CellFields and FEFunctions so should be alright.