Simplify stride calculations in loglocator.

[anntzer]

These are just algebraic manipulations:

In classic mode, we want the smallest stride so that

numdec/stride+1 <= numticks

i.e.

stride >= numdec/(numticks-1)

so stride is the ceil() of the RHS (and at least 1, to handle the case
vmin==vmax).

In nonclassic mode, we want the smallest stride so that

numdec//stride+1 <= numticks, i.e. numdec//stride < numticks

(everything is integer), i.e.

numdec < numticks*stride- 1

i.e.

stride > (numdec+1)/numticks

so stride is 1 + the floor() of the LHS.

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way