Rough out Datac class

I included documentation of how I expect `Datac` objects to be
initially instantiated. The `__init__` method sets the object
attributes with the incoming data in a way that is not read-only.
Nothing yet happens to the `calc_method` argument.
Draft method to calculate ordinates
Remove test file

'Datac' should have been capitalized and wasn't.
Disable generate_ordinates for testing
Import Datac class into top-level namespace
Add type checks of args during instantiation
Add some tests of instantiation
Add some API tests

These tests test if params, abscissae, and abscissa_name are
read-only.
Make params, abscissae, abscissa_name read only
Factor common variables out of individual tests
Add initialization code for hand-testing
Test ordinates attribute of Datac is read only
Make ordinates attribute of Datac read-only

Closes #3.
Add test for existance of ordinates property

The `ordinates` property of a `Datac` instance should exist if a class
method was passed to the constructor.
Calculate ordinates if instantiated with method
Fix functionally benign typo
Test calc_method is callable
Raise exception if calc_method fails when called

The exception is raised if an object cannot be created from
`calc_method.im_class`.
Test set calc_method after default instantiation
Test ordinants exists after setting calc_method