Fixed run-k-points bug by adding change-k-point.

[acerjan]

Slight update to documentation to remind users to set the k-point prior to adding flux / energy regions.

Resolves #770.

[stevengj]

I updated it with a few fixes:

• Once you call change-k-point! you no longer need to call init-fields since the fields are already initialized.
• In Python, we don't call output_epsilon by default anymore in other run functions, so that whole block can be eliminated.
• In Python, the functions add_flux etcetera are now "lazy" (that is, they set up an initialization that is actually performed only when the run begins), so we can remove the language in the manual about only calling them after setting parameters.

[stevengj]

Will merge once tests are green again.

[acerjan]

When I was testing change-k-point! without init-fields in scheme, the output-epsilon call threw errors complaining that the fields had not been initialized. That's why I left in init-fields.

[stevengj]

Oh, you are right, restart-fields only re-initializes the fields if they have already been initialized. I updated it to call init-fields, but with a guard to ensure that it only re-allocates the fields if needed.