split elementary functions

[Nelson-numerical-software]

We could move operators to an "operators" module.

horzcat,
power,                      
ismember,
rdivide,
all,
ldivide,                     
and, 
le,
subsasgn,                   
any,
lt,
subsref,
minus,
times,                      
mldivide,
transpose,                 
mpower,
uminus,
mrdivide,                      
mtimes,
unique,                     
ne,                 
colon,
not,
uplus,                      
ctranspose,
vertcat,                   
eq,
or,
xor,                        
ge,
gt,
plus

split will allow to build faster (at least on windows)
elementary_functions is too big (2.34 Mo)