Add an option to control Amr regrid interval when max_level==0

[cityadmirer]

Summary

Add Amr::regrid_level0_int option to control regrid interval when max_level==0. It has default value -1 and takes effect only when force_regrid_level_0==false thus should not change the current behavier. Amr::timeStep now reads and sets level_count when regrid max level 0

Additional background

Before this PR, when max_level==0, regrid_int is not used and force_regrid_level_0 is the only way to perform regrid. However, the force_regrid_level_0 option regrid at every step without interval control.

The proposed changes:

• fix a bug or incorrect behavior in AMReX
• add new capabilities to AMReX
• changes answers in the test suite to more than roundoff level
• are likely to significantly affect the results of downstream AMReX users
• include documentation in the code and/or rst files, if appropriate