Use math expression to enhance performance

[huiyuxie]

Current implementation rely on function isequal to avoid control flow divergence. The benchmarks showed that this function can be replaced by math expression to enhance the performance.

For example,

# `side` equals either 1 or 2
boundaries_u[1, j, k] = u[j, size(u, 2), element] * isequal(side, 1) # check if `side` equals 1
boundaries_u[2, j, k] = u[j, 1, element] * isequal(side, 2) # check if `side` equals 2

can be easily replaced by

# `side` equals either 1 or 2
boundaries_u[1, j, k] = u[j, size(u, 2), element] * (2 - side) 
boundaries_u[2, j, k] = u[j, 1, element] * (side - 1) 

This also applies to other similar situations.

[huiyuxie]

Address part of #22

[huiyuxie]

The isequal performs better in 3D case but remains a little bit worse than math expression in 1D and 2D cases. So here we use hybrid of isequal function and math expression.