Fixed file mode for reading pickle tiles
Added function to properly convert strings to long doubles
Update toa.py

Corrects two bugs: the first given by using "is" instead of "==" to compare a variable and a string, and the second given by using the attribute "tdb" for an astropy column instead of the times it contains.
Merge pull request #7 from matteobachetti/patch-3

Update toa.py.  Thanks, Matteo!
Merge branch 'master' of github.com:nanograv/PINT