add a helper function that returns the fine grid dx

[zingale]

this is for the case when we need dx before the level hierarchy is setup, like in problem_initialize.H

PR summary

PR motivation

PR checklist

• test suite needs to be run on this PR
• this PR will change answers in the test suite to more than roundoff level
• all newly-added functions have docstrings as per the coding conventions
• the CHANGES file has been updated, if appropriate
• if appropriate, this change is described in the docs

[zingale]

closes #669

[maxpkatz]

I think I am confused about whether we still need this. At this point, problem_initialize() is called in Castro::initMFs(), which is done in the Castro constructor after the Amr object has been created, so the geometry object on all levels should already be populated.

[zingale]

Oh. Then how do I get the fine level dx?

[WeiqunZhang]

If you make problem_initialize a member function of Castro, you can get the Geometry on the max_level with this->parent->Geom(this->parent->maxLevel()). You can also change the interface of the function and pass this to problem_initialize(Castro const*), if you don't want to make it a member function. It's basically the same thing. A member function has this as a hidden argument.

[WeiqunZhang]

Another possibility is to store a pointer to Amr after it's created in main.cpp as a global variable. Then you can access it from anywhere.

[zingale]

closing in favor of #2608