feature request: Implement tune.block.spls()

[Max-Bladen]

Is your feature request related to a problem? Please describe.
This is a function that is lacking and seems like an obvious addition to the package. This would increase the homogenity across tuning the various methods within mixOmics

Describe the solution you'd like
In the same structure as the other tuning functions (such as tune.block.splsda() and tune.spls()), create a function which can output the optimal keepX values for numerous blocks