Refactor some code for understandability

[micahjsmith]

Hello, I spent a fair amount of time reading through the code of jsonmodels to understand how it worked. It employs a fair amount of interesting python constructs, like __get__ and __set__ methods of BaseField and a healthy use of metaclasses and class methods.

To help myself understand what was going on, I found myself making small changes to the code for readability and interpretability. This includes

• consistently use if/else to make control flow explicit
• fix typo of structure_name (still leaving structue_name for backwards compatibility)
• use more explicit names for iteration over class attributes

  
  # original
  for attr in dir(cls):
  clsattr = getattr(cls, attr)

new

for attr_name in dir(cls): field = getattr(cls, attr_name)

- try to simplify logic of field initialization by adding an `_initialized` attribute to `BaseField` and guarding against re-initialization
- make `Base.get_field` a classmethod, which avoids a linear search through the entire set of fields on each access

I thought other people might benefit from these small changes. Hope this is helpful, let me know what you think!

[micahjsmith]

Hi @beregond , any thoughts on this PR?

[micahjsmith]

Hi @beregond any thoughts on this PR?