Basic syntax: reflective, formative, interactions

[soumyaray]

DO NOT MERGE YET!

Finished:

• reflective and formative items
• skeleton usage info
• skeleton readme (through .rmd)
• demo files

Doing:

• allow basic interaction terms

[soumyaray]

We can now create fairly feature rich PLS models with the syntax:

• reflective and formative factors
• multi-item and single-item factors
• interaction terms