Stop removing leading comments and new lines

[BoPeng]

SoS Notebook is pretty loose on "header" in the sense that it ignores leading comments (# lines) and empty lines. So one can write

# this step does this
%expand

This however does not work if we are to include kernels and languages with significant # (e.g. #57 ). I think it makes sense to require SoS magics to start from the first line, and any new line would stop SoS from processing the text as header. This also solves the problem with #33 because we can simply do

%sosmagic

%cellmagic

[BoPeng]

116ac4a