Adds 3 basic implementations of approximations for gp.Latent (#3)

* cp over hsgp class

* copy over pymc gitignore

* add basic DTC/projectedprocess implementation

* add KarhunenLoeveExpansion, add notebook

* Add conditional draws to latent_approx notebook

Co-authored-by: lucianopaz <[email protected]>
Co-authored-by: Osvaldo A Martin <[email protected]>

Author: 3 people