Add ALT Mean Only to specifications

[fl-modex]

Following OasisLMF/ktools#357 / OasisLMF/OasisLMF#1378

[johcarter]

Although a separate calculation may be used to generate mean only ALT, this report could still be stored in existing ORD PALT format without the Standard Deviation column, or with it set to zero. You could have all summary levels with and without stdev in one PALT file. One for discussion perhaps.