add missing <optional> header to src/projects/mts/kmer_multiplicity_counter.cpp #96

Job	Run time
🚧 Build SPAdes	1m 17s
Mate-pairs A.mirum	5m 9s
Mate-pairs L.gasseri	4m 1s
Isolated E.coli is220	7m 30s
Isolated L.gasseri	4m 53s
Nextera mate-pairs K.pneumoniae	1m 55s
Nextera mate-pairs R.sphaeroides	1m 26s
Mate-pairs S.smaragdinae quaked	7m 6s
	33m 17s

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