[WIP] append to sequence file output

include/seqan3/io/sequence_file/output.hpp

@@ -133,25 +133,31 @@ class sequence_file_output
     /*!\brief Construct from filename.
      * \param[in] filename      Path to the file you wish to open.
      * \param[in] fields_tag    A seqan3::fields tag. [optional]
+     * \param[in] openmode      The open mode for the file stream. [optional]
      *
      * \details
      *
      * In addition to the file name, you may specify a custom seqan3::fields type which may be easier than
      * defining all the template parameters.
      *
+     * You may also specify the open mode for the file stream. `std::ios_base::out` and `std::ios::binary` will always
+     * be added regardless of the given open mode.
+     * `std::ios_base::app` can be used to open the file in append mode.
+     *
      * ### Compression
      *
      * This constructor transparently applies a compression stream on top of the file stream in case
      * the given file extension suggests the user wants this.
      * See the section on \link io_compression compression and decompression \endlink for more information.
      */
     sequence_file_output(std::filesystem::path filename,
-                         selected_field_ids const & SEQAN3_DOXYGEN_ONLY(fields_tag) = selected_field_ids{}) :
+                         selected_field_ids const & SEQAN3_DOXYGEN_ONLY(fields_tag) = selected_field_ids{},
+                         std::ios_base::openmode openmode = std::ios_base::out | std::ios::binary) :
         primary_stream{new std::ofstream{}, stream_deleter_default}
     {
         primary_stream->rdbuf()->pubsetbuf(stream_buffer.data(), stream_buffer.size());
         static_cast<std::basic_ofstream<char> *>(primary_stream.get())
-            ->open(filename, std::ios_base::out | std::ios::binary);
+            ->open(filename, openmode | std::ios_base::out | std::ios::binary);
 
         if (!primary_stream->good())
             throw file_open_error{"Could not open file " + filename.string() + " for writing."};