IgtMix dynamic array extension bug

[chmarti1]

There is a bug in IgtMix.__setitem__() that causes incorrect behavior when increasing the dimension of a mixture array dynamically.

>>> import pyromat.igtools as igt
>>> import pyromat as pm
>>> pm.config['unit_matter'] = 'kmol'
>>> mix = igt.IgtMix('3N2 + 2O2')
>>> mix['N2'] = [[3,2],[1,0]]
>>> print(mix)
N2 : array([[3., 2.],
       [1., 0.]])
O2 : array([[3., 3.],
       [2., 2.]])

The original O2 quantity was 2 kmol, and it should have been broadcast to be 2 kmol at all mixture array elements. The behavior seems to be unique to higher dimension arrays.

[chmarti1]

Unfortunately, this needs a bit of a think w.r.t. how IgtMix handles array resizing. The problem is created because I'm supporting a method for resizing mixture arrays that Numpy doesn't, so the built-in tools don't quite work the way you'd like them to.

I think the fix is going to be disallowing this approach, but instead providing a resizing method that provides the same functionality.