ebnm_check_fn() should check that function can handle x = c(-10, 0, 10)

[willwerscheid]

this is the test value of x used by flashier. If it cannot handle it then a message should be printed with a suggested solution, e.g., include

if (length(x) == 3){ ### just to satisfy check of custom function
    return (ebnm_flat(x))
  }

[willwerscheid]

also update custom ebnm tutorial to mention this issue