Add Null mapping

[bernhardmgruber]

The Null mappings maps all elements to nothing. Writing data through a reference obtained from the Null mapping discards the value. Reading through such a reference returns a default constructed object.

[codecov]

Codecov Report

Merging #442 (17127f9) into develop (0d0efbe) will decrease coverage by 0.01%.
The diff coverage is 93.33%.

@@             Coverage Diff             @@
##           develop     #442      +/-   ##
===========================================
- Coverage    99.22%   99.20%   -0.02%     
===========================================
  Files           56       58       +2     
  Lines         5150     5165      +15     
===========================================
+ Hits          5110     5124      +14     
- Misses          40       41       +1     

Impacted Files	Coverage Δ
tests/mapping.Null.cpp	90.90% <90.90%> (ø)
include/llama/mapping/Null.hpp	100.00% <100.00%> (ø)

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[bussmann]

Next up: Identity mapping.

[bernhardmgruber]

Next up: Identity mapping.

I am not sure what you have in mind here. How can I have an identity mapping, when domain (data space) and codomain (blob nr/offset space) are different sets?