Add auto-time-update flag to NIXpy file

[hkchekc]

Refer to the issue #368

I added a flag for timestamp updating when the file is opened. 2 functions (time_auto_update_on() and time_auto_update_off()) are added for altering the flag after the file is opened.

The current policy is to update the time whenever a property of an Entity or File is changed and when the time_auto_update flag is True. I do not put the flag in the force_update_at() function because it is more flexible this way. Note that the update of metatdata will not change the update_at() time anyway.

The force_update_at() in creation is not affected.

As of the performance issue, the time used for changing the expansion_origin for data_array 100,000 times is:

EDITED:

always update (without a flag): around 45s
flag = True: around 45s (possibly slower for sections or property)
flag = False: around 20s
do not provide an auto_update possibility at all: around 20s (slightly less than flag = False)

[coveralls]

Coverage decreased (-0.09%) to 89.541% when pulling 4311162 on hkchekc:time into 50a156e on G-Node:master.

[coveralls]

Coverage decreased (-0.3%) to 89.328% when pulling f5b019c on hkchekc:time into 50a156e on G-Node:master.

[achilleas-k]

I don't think the auto-update setting should be stored in the file. This should be a session option, much like the compression flag and the open mode.

I also don't see the reason for having two methods, one for turning it off and one for on. Why not a property nixfile.auto_update_timestamps = {True,False}?

[achilleas-k]

Failing test is due to mismatching File Format versions with NIX breaking a compatibility test. Will be fixed by #386.