BUG: Fixed object type missmatch in SymLogNorm

Currently SymLogNorm accepts both linthresh and vmin, vmax with their
native dtype, meaning that the contents of one object cannot be updated
with another due to a mismatch of types.
Merge pull request #2428 from cpelley/symlognorm_fix

BUG: Fixed object type missmatch in SymLogNorm
BUG: Fixed object type missmatch in SymLogNorm

Currently SymLogNorm accepts both linthresh and vmin, vmax with their
native dtype, meaning that the contents of one object cannot be updated
with another due to a mismatch of types.
Merge remote-tracking branch 'upstream/v1.3.x'
BUG Raise exception for invalid input

Arrays of non-uint8 types were assumed to be in 0..1 range, resulting in
the small bits being showing when this was not true. Now, an explicit
exception is raised.

Closes issue #2499