[Encoders] Handle exception `samtools mpileup` output when searching for insertions in  SummaryEncoder 

When searching for the inserted positions in the pileup strings read from the `.pileup` file (which was generated by samtools), an exception occurs in the `find_insertions()` methods when a few pileup reads do not have a digit after the '+' sign, e.g.:
` TTTtttTt^+t`

Was tested on `m54328_180927_224427`
