Add new DecFileParser class, with test suite

[eduardo-rodrigues]

With this new DecFileParser class I was able to trivially parse and check the master LHCb DECAY.DEC file:

from decaylanguage.dec.dec import *
import time

start = time.time()
p = DecFileParser('decaylanguage/data/DECAY_LHCB.DEC')
p.parse()
end = time.time()
print('Parsing time = %s seconds.' % (end-start))

The massive file is nicely parsed in under 2 seconds :-).

And, BTW, I found a little bug in the file - duplication of decay definition for the Sigma(1775)0 particle! LHCb simulation group about to be informed ...

[eduardo-rodrigues]

I will be adding a bit more doc, in particular. But the code is otherwise up for discussion, @henryiii.

[eduardo-rodrigues]

@henryiii, this is now ready for review, though I will still add some more documentation, and am also thinking about an extra method to display a la dot-file a decay chain, for example.

[coveralls]

Coverage remained the same at 0.0% when pulling abb8e20 on eduardo-decfileparser-intro into 6369662 on master.

[henryiii]

Can you rebase to see if this starts passing? Looks good otherwise. I might recommend a __slots__ just for clarity.

[henryiii]

More docs and things would be good, though I'm okay to go ahead and merge, then add to it afterwards in a new PR.

[eduardo-rodrigues]

Thanks a lot for the look and the improvements/fixes.

I've just rebased.

[eduardo-rodrigues]

Forgot to say: I do want to add more docs, and tests, though I put a significant amount of it already. I will merge, then, as this PR is rather large and contains some nice additions.

Thanks.