Low level optimisations

[JordiManyer]

We noticed weird allocations happening, related to the creation of Field-related structures.

Log of changes:

• AffineMap renamed to AffineField.
• New AffineMap <: Map, doing the same as AffineField without struct allocation.
• New ConstantMap <: Map, doing the same as ConstantField without struct allocation.

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 89.05%. Comparing base (3b87738) to head (e279910).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Fields/AffineMaps.jl	95.00%	1 Missing ⚠️
src/Fields/ApplyOptimizations.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1043      +/-   ##
==========================================
+ Coverage   89.04%   89.05%   +0.01%     
==========================================
  Files         197      197              
  Lines       23887    23902      +15     
==========================================
+ Hits        21270    21286      +16     
+ Misses       2617     2616       -1     

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