Treat the case where `maxpts > len(data)`

According to the document, the skymap is generated from the entire data when maxpts is larger than the length of data, which is not the case because np.random.choice(a, b, replace=False) returns the following error when a < b.

>>> import numpy as np
>>> np.random.choice(100, 1000, replace=False)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "mtrand.pyx", line 1046, in mtrand.RandomState.choice (numpy/random/mtrand/mtrand.c:7591)
ValueError: Cannot take a larger sample than population when 'replace=False'