spca_randtest - single eigenvalue test

[valemon]

Hello! the current spca_randtest only gives results for global and local patterns but does not give p-values for single eigenvalues which is otherwise included in the originally published script (BMC Bioinformatics paper). Would you guys have the time to implement it? I think Thibaut has the initial code that I had produced for the publication if it is of any help. Thanks very much and keep up the good work!

[thibautjombart]

Hi Vale, can you PR the changes?