Add assign feature

[Raphael-CV]

Add assign feature (cf #23). When there are assignments, variables and values are stored to be displayed at the end. These lines:

@latexify.with_latex
def f(x):
    res = x**2
    return res

are the same as:

@latexify.with_latex
def f(x):
    return x**2

[odashi]

Could you add several unit tests to cover the feature proposed in this change?

[odashi]

I think this doesn't work correctly if the function has nested defs:

def foo(x):
    y = 42
    def bar(x):
        y = 123
    return y

[odashi]

At this moment, I guessed that it would be better to raise an error when the parser found nested functions or classes (all things that define an inner namespace).

[odashi]

Some concerns around this:

• It may cause exponential expansion. An extreme case:

  def fn(x):
      a = x + x
      b = a + a
      c = b + b
      return c

  This may return $x+x+x+x+x+x+x+x$.

• If the assigned string has some structure, we need to add appropriate parentheses around the string.

[Raphael-CV]

To avoid exponential expansion, I suggest to add an option to activate or not the assignment.
For example:

@latexify.with_latex(assign_mode=False)
def h(x):
    a = abs(x)
    b = math.exp(math.sqrt(x))
    return a * b

will return a = \left|{x}\right| \\ b = \exp{\left({\sqrt{x}}\right)} \\ \mathrm{h}(x) \triangleq ab

[Raphael-CV]

There is also a problem when assignment refers to several numbers with a product like:

@with_latex
def f(x):
    a = 1
    b = 2
    return a * b

return \mathrm{f}(x) \triangleq 12

[odashi]

Looks good. I think reduce_assignments is good for the option name (assign_mode boolean option is somewhat confusing to users)

[Raphael-CV]

It seems I break some tests ahah, I am going to correct this

[Raphael-CV]

There are still some problems with parenthesis. To avoid some problems like this function:

@with_latex(reduce_assignments=True)
def f(x):
    y = 1 + x
    return y ** 2

that would returns $\mathrm{f}(x) \triangleq 1 + x^{2}$, I added parenthesis around each assigned functions in order to have $\mathrm{f}(x) \triangleq \left( 1 + x \right)^{2}$.

But there are some cases where it is useless and unsightly, for example:

@with_latex(reduce_assignments=True)
def f(x):
    y = math.exp(x)
    return y ** 2

returns $\mathrm{f}(x) \triangleq \left( \exp{\left({x}\right)} \right)^{2}$.

[odashi]

I think it is okay. redundant parentheses are general problems on this library and we need some entire solution on this, but it is somewhat off-topic of this pull request.