WIP: Data checker rework

[mtlam]

From our discussions, this PR is intended to revamp the par checker into a more general data checker. This is being worked on in conjunction with the summary PDF rework (#145) with some items from the old PDF moving here. The main part of this code is heavily based on David Kaplan's ngchecker implementation for this purpose.

This code contains an abstract DataChecker class and multiple sub-classes that inherit from this one:

• ParChecker: checks the required spin/astrometric parameters internally, and then calls the following two classes:
  • NameChecker: checks the pulsar name
  • BinaryChecker: checks the binary parameters
• EpochChecker: checks the epoch centering
• TOAChecker: checks the TOA versioning, allows for bad ranges to be defined here as well.

Includes data_checker.py, test_data_checker.py, and another 12.5-year data set tim file for unit testing. This also removes par_checker.py and test_par_checker.py as obsolete.

To-dos:

• Unit tests for TOAChecker.
• Verify desired behavior on several of the other checkers.
• Verify use of full tim files, or strip them down to a smaller size for unit tests.