[MISC] Switch from uppercase seqan3::field names to lower case.

[smehringer]

Resolves #1422

[codecov]

Codecov Report

Merging #1421 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1421   +/-   ##
=======================================
  Coverage   97.59%   97.59%           
=======================================
  Files         232      232           
  Lines        8825     8825           
=======================================
  Hits         8613     8613           
  Misses        212      212

Impacted Files	Coverage Δ
.../seqan3/io/sequence_file/output_format_concept.hpp	100% <ø> (ø)	⬆️
include/seqan3/io/structure_file/input.hpp	100% <ø> (ø)	⬆️
...clude/seqan3/io/alignment_file/format_sam_base.hpp	98.1% <ø> (ø)	⬆️
include/seqan3/io/sequence_file/format_fastq.hpp	98.41% <ø> (ø)	⬆️
include/seqan3/io/sequence_file/format_embl.hpp	97.46% <ø> (ø)	⬆️
.../seqan3/io/structure_file/input_format_concept.hpp	100% <ø> (ø)	⬆️
include/seqan3/io/sequence_file/input_options.hpp	100% <ø> (ø)	⬆️
...seqan3/io/alignment_file/output_format_concept.hpp	100% <ø> (ø)	⬆️
include/seqan3/io/alignment_file/header.hpp	100% <ø> (ø)	⬆️
include/seqan3/io/sequence_file/format_fasta.hpp	97.26% <ø> (ø)	⬆️
... and 13 more

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[Irallia]

Looks perfect, I have cross-checked all the field names.

[rrahn]

Do we really need this?

[smehringer]

Well since this is actually already used rather often, I think it is important to provide a clear message to the user. Additionally this does not hurt much, does it?

[smehringer]

@rrahn Do you agree?

[rrahn]

yes it is fine.

[rrahn]

Only one open question.