fix edge case in marker value import

[dominikbach]

Fixes #727, an edge case reported in https://github.com/bachlab/PsPM/discussions/719.

Changes proposed in this pull request:

• generalise syntax for [S(:).field] for cases in which numel(S(i).field) ~= 1 in a new function pspm_struct2vec
• call this in several import functions
• fix a bug that leads to markerinfos.values being char rather than num.