Create an ab initio parse method.

[MicahGale]

Description

This feature adds the ability to parse MCNP inputs arbitrarily. All MCNP_Object now accepts and parses str. This also introduces MCNP_Problem.parse which is a convenience function that just parses, and goes. This also adds the ability to pass a number to a constructor to simplify the build.

Fixes #88

Checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[MicahGale]

Also an open question is: if this should ship for 0.6 or 1.0

[MicahGale]

To document pretty_str or not?