Shielded libstempo dependencies if only pint installed

[Hazboun6]

This is Pull Request to deal with Issue #183. I've changed the code in enterprise.Pulsar() so that libstempo is no longer required if user hasPINT installed.

[paulthebaker]

While you're at it you could replace the list(filter( ...)) with list comprehensions. I think this is equivalent, but I haven't tested:

[isinstance(x, t2.tempopulsar) for x in args]

[Hazboun6]

I think what you have written will return a list of booleans, but the list(filter()) returns a list of objects.

[paulthebaker]

Okay. To get a list of objects, you want:

[x for x in args if isinstance(x, t2.tempopulsar)]

I sort of tested it... This gives me a list of ints:

args = [0.0, 1, 2, 3.0, 4, 5.0]
[x for x in args if isinstance(x, int)]

I find list comprensions to be much more readable than map/filter/reduce statements.
For this case it doesn't matter, but lambda statement are usually way slower than other options.

[Hazboun6]

As @paulthebaker suggested elsewhere, there are many instances of list(filter()) throughout the code. This may be taken up at a later time.