SaveAscii spectra indexing

[Raquel Alvarez Banos]

To reproduce:

• Load ​https://github.com/mantidproject/mantid/blob/master/Test/AutoTestData/GEM38370_Focussed_Legacy.nxs, dataset has 6 spectra ranging from 2 to 7
• Execute SaveAscii with SpectrumList property set to "6,7". An error message indicating an "Inconsistent spectra list" is issued.

Also:

• Load any dataset, for instance ​https://github.com/mantidproject/mantid/blob/master/Test/AutoTestData/HET15869.raw, with spectra ranging from 1 to 2584.
• Try saving the last spectrum with SaveAscii. The "Inconsistent spectra list" error message is issued.

Spectra ID should range from 1 to N instead of 0 to N-1.

[Nick Draper]

Moved to R3.5 at the R3.4 code freeze

[Stuart Campbell]

This ticket has been transferred to github issue ​11364