fix edge case in marker value import

[dominikbach]

Fixes #727, an edge case reported in #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.

[teddphil]

I tested this function with two-dimensional structure array, and I can see it works well with this function. The only thing is, this function will provide a 1xN vector v, where N denotes the number of elements in the structure array. If it is a two dimensional structure array with size p x q, the output will be 1 x p*q vector. If this is expected, I can merge this PR directly. Thank you.

[teddphil]

Testing code:

field = 'f';
value = {'some text','a';
[10, 20, 30],'b';
magic(5),3};
S = struct(field,value);
v = pspm_struct2vec(S, 'f')