Use bitwise xor between masks in remove lines
Use bitwise xor between masks in remove lines

The line removal function was previously subtracting to boolean masks,
which is no longer supported in NumPy 1.14 (raises a `TypeError`
instead). Here we switch over to bitwise xor, which provides the same
result and is backwards compatible. Should fix this bug.

nanshe-org/nanshe,jakirkham/nanshe,DudLab/nanshe,nanshe-org/nanshe,DudLab/nanshe,jakirkham/nanshe