Document `State` (and make it represented as an int)

[brunobeltran]

The State should only account for total number of sites bound and we can do the combinatorics for how this affects the energy under the hood.

[brunobeltran]

This will superficially be fixed by #22 and #24.

We should probably, however, add some wrapper classes that document how this should be dealt with more carefully, like:

  13 class State(Enum):
  12     """Defines the chemical state of a given bead for a single mark."""
  11
  10     pass
   9
   8
   7 class HP1_State(State):
   6     """HP1 can be bound to none, either, or both nucleosome tails."""
   5
   4     NONE = 0
   3     LBOUND = 1
   2     RBOUND = 2
   1     BOTH = 3
   0

but with hella docstring.