Interval overlaps

Code like below was failing because user intervals overlapped with default bad time intervals:
```
import Ska.engarchive.fetch as fetch
import Chandra.Time
from Ska.Matplotlib import plot_cxctime
 
time_start = Chandra.Time.DateTime('2008:001:00:00:00.000').secs
time_stop  = Chandra.Time.DateTime('2014:096:00:00:00.000').secs
fetch.read_bad_times('bad_times.dat')
 
bias1_daily = fetch.MSID('aogbias1', time_start, time_stop, stat='daily')
bias1_daily.filter_bad_times()

/proj/sot/ska/test/arch/x86_64-linux_CentOS-5/lib/python2.7/site-packages/Ska.engarchive-0.28-py2.7.egg/Ska/engarchive/fetch.pyc in _get_table_intervals_as_list(table)
    303         # Check for overlaps
    304         if any(i0[1] > i1[0] for i0, i1 in izip(intervals[:-1], intervals[1:])):
--> 305             raise ValueError('Input intervals overlap')
    306
    307     return intervals
 
ValueError: Input intervals overlap
 ```
This changes the behavior so that no overlap checking is done in this case.