Cigar operation (seqan#2388)

* [MISC] deprecate seqan3::cigar_op and add changelog

* Apply suggestions from code review

CHANGELOG.md

@@ -98,8 +98,13 @@ If possible, provide tooling that performs the changes, e.g. a shell-script.
 
 #### Alphabet
 
-* Removed seqan3::char_is_valid_for requirement from seqan3::writable_alphabet and seqan3::writable_constexpr_alphabet
+* Removed seqan3::char_is_valid_for requirement from seqan3::writable_alphabet and
+  seqan3::detail::writable_constexpr_alphabet
   ([\#2337](https://github.com/seqan/seqan3/pull/2337)).
+* Removed seqan3::cigar_op, please use seqan3::cigar::operation instead
+  ([\#2388](https://github.com/seqan/seqan3/pull/2388)).
+* The literal 'M'_cigar_op was renamed to 'M'_cigar_operation
+  ([\#2388](https://github.com/seqan/seqan3/pull/2388)).
 
 #### Argument Parser
 

include/seqan3/alphabet/cigar/cigar.hpp

@@ -232,6 +232,12 @@ inline cigar::operation operator""_cigar_operation(char const c) noexcept
 {
     return cigar::operation{}.assign_char(c);
 }
+
+//!\deprecated Please use seqan3::""_cigar_operation instead.
+SEQAN3_DEPRECATED_310 inline cigar::operation operator""_cigar_op(char const c) noexcept
+{
+    return cigar::operation{}.assign_char(c);
+}
 //!\}
 
 } // namespace seqan3

include/seqan3/alphabet/cigar/cigar_op.hpp

@@ -0,0 +1,26 @@
+// -----------------------------------------------------------------------------------------------------
+// Copyright (c) 2006-2020, Knut Reinert & Freie Universität Berlin
+// Copyright (c) 2016-2020, Knut Reinert & MPI für molekulare Genetik
+// This file may be used, modified and/or redistributed under the terms of the 3-clause BSD-License
+// shipped with this file and also available at: https://github.com/seqan/seqan3/blob/master/LICENSE.md
+// -----------------------------------------------------------------------------------------------------
+
+/*!\file
+ * \brief [DEPRECATED] Introduces the seqan3::cigar_op alphabet.
+ * \author Joshua Kim <joshua.kim AT fu-berlin.de>
+ * \deprecated This header will be removed in 3.1.0; Please \#include <seqan3/alphabet/cigar/cigar.hpp>
+ *             instead.
+ */
+
+#pragma once
+
+#include <seqan3/alphabet/cigar/cigar.hpp>
+
+namespace seqan3
+{
+//!\deprecated Please use seqan3::cigar::operation instead.
+using cigar_op SEQAN3_DEPRECATED_310 = seqan3::cigar::operation;
+} // namespace seqan3
+
+SEQAN3_DEPRECATED_HEADER(
+   "This header is deprecated and will be removed in SeqAn-3.1.0; Please #include <seqan3/alphabet/cigar/cigar.hpp> instead.")