Add .gitignore for .swp and .pyc files.
Add a method for copying formula and partial initialization.

Reasons:
- paralelization requires both copy and partial initialization of a
  formula.
Merge setting values and simplifying formula into new method.

Reasons:
- simplicity
- manipulation of formula produces formulas or constants represented as
  objects, SAT should interpret those objects as True or False.
Add code into src directory.