Fix ProForma writer for ranges bearing multiple modifications (§4.5) - #116

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Cannot convert null literal to non-nullable reference type.

Possible null reference return.

Possible null reference return.

Cannot convert null literal to non-nullable reference type.

Cannot convert null literal to non-nullable reference type.

'IIsotopicDistribution.CreateChargedDistribution(int, bool)' is obsolete: 'Use CreateChargedDistribution(int charge, double chargeCarrier) instead.'

Cannot convert null literal to non-nullable reference type.

Cannot convert null literal to non-nullable reference type.

Cannot convert null literal to non-nullable reference type.

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