Feature 54 add pca

[asizemore]

Resolves #54

Adds pca method. This method wraps prcomp. Not much else to say!

To do:

• main function
• basic tests
• test with messy data
• Change the output to a ComputeResult. Originally i wanted just a data.table for simplicity. But after starting to wire it up it'd be slightly more helpful if the pca output had some extra info.

[asizemore]

@bobular i decided to change the output a bit, so i removed you from the reviewers list. I'll update here when it's ready again.

[asizemore]

@bobular ready for you to take a look! Not in a rush.
I'll be using this branch to draft a new viz plugin on the backend.