clean up project() a bit

[rpruim]

Things to do:

• move some arguments out of the generic
• type = "length" results in something that is signed, this should be fixed
• type = "coef" or type = "scalar" should be added to get the multiplier
• documentation could use a read through and update.

[rpruim]

Now also handling projection onto the 0 vector better (return a 0 vector or 0 depending on type).

[rpruim]

Make sure fastR code still works after changes.