WIP: resize with local mean by non-integer size

[shoyer]

Not for serious review

I'm making this PR mostly to clarify that it is OK to reuse this code under skimage's license, e.g.,
inside #4201

[pep8speaks]

Hello @shoyer! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file skimage/transform/_warps.py:

Line 20:1: E303 too many blank lines (3)
Line 20:1: E402 module level import not at top of file
Line 24:3: E111 indentation is not a multiple of four
Line 25:3: E111 indentation is not a multiple of four
Line 26:3: E111 indentation is not a multiple of four
Line 27:3: E111 indentation is not a multiple of four
Line 29:1: E302 expected 2 blank lines, found 1
Line 31:3: E111 indentation is not a multiple of four
Line 43:3: E111 indentation is not a multiple of four
Line 44:3: E111 indentation is not a multiple of four
Line 45:3: E111 indentation is not a multiple of four
Line 47:3: E111 indentation is not a multiple of four
Line 48:3: E111 indentation is not a multiple of four
Line 49:3: E111 indentation is not a multiple of four
Line 51:3: E111 indentation is not a multiple of four
Line 53:1: E302 expected 2 blank lines, found 1
Line 54:5: E125 continuation line with same indent as next logical line
Line 55:3: E111 indentation is not a multiple of four
Line 65:3: E111 indentation is not a multiple of four
Line 68:3: E111 indentation is not a multiple of four
Line 72:3: E111 indentation is not a multiple of four
Line 73:3: E111 indentation is not a multiple of four
Line 74:3: E111 indentation is not a multiple of four
Line 75:3: E111 indentation is not a multiple of four
Line 77:1: E302 expected 2 blank lines, found 1
Line 80:3: E111 indentation is not a multiple of four
Line 99:3: E111 indentation is not a multiple of four
Line 100:3: E111 indentation is not a multiple of four
Line 102:7: E111 indentation is not a multiple of four
Line 105:3: E111 indentation is not a multiple of four
Line 106:3: E111 indentation is not a multiple of four
Line 111:3: E111 indentation is not a multiple of four
Line 116:3: E111 indentation is not a multiple of four
Line 121:3: E111 indentation is not a multiple of four
Line 127:3: E111 indentation is not a multiple of four
Line 132:3: E111 indentation is not a multiple of four
Line 137:3: E111 indentation is not a multiple of four
Line 144:3: E111 indentation is not a multiple of four
Line 149:3: E111 indentation is not a multiple of four
Line 154:3: E111 indentation is not a multiple of four
Line 160:3: E111 indentation is not a multiple of four
Line 166:3: E111 indentation is not a multiple of four
Line 172:3: E111 indentation is not a multiple of four
Line 174:7: E111 indentation is not a multiple of four