add sanity check to n.pca.max to default to max value if vector

[pdimens]

This is an extra sanity check to make sure #322 cannot happen

[zkamvar]

Thank you for the contribution! I will need to add a test in https://github.com/thibautjombart/adegenet/blob/master/tests/testthat/test_xval.R to make sure it does not regress.

Could you add your name to the DESCRIPTION file with a "ctb" role?

[pdimens]

Added