Fix dict to live point bug

[mj-will]

Fix a bug when calling dict_to_live_points with a dictionary containing floats.

Example

Python 3.8.5 (default, Sep  4 2020, 07:30:14) 
[GCC 7.3.0] :: Anaconda, Inc. on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from nessai.livepoint import dict_to_live_points
>>> d = dict(a=1.0, b=2.0)
>>> dict
dict(                 dict_to_live_points(  
>>> dict_to_live_points(d)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/michael/git_repos/nessai/nessai/livepoint.py", line 130, in dict_to_live_points
    N = len(list(d.values())[0])
TypeError: object of type 'float' has no len()
>>>