Remove IPython extension as Pandas can be used for HTML display

This removes the ipython.py file, which previously served as an
IPython extension. The same functionality can be achieved by using
a Pandas DataFrames which can be accessed via the dframe property.
Remove Indexed and LexSorted classes

Pandas offers the same functionality as the removed classes by
operating on DataFrame objects accessed via the dframe property.
Turned constant_keys, varying_keys and constant_items into properties
Removal of the applying decorator

Accumulation functionality can be achieved with Pandas or manually.
 Factor out dynamic specifiers (experimental) into a separate module

This refactor has simplified the core and allowed the show method of
both specifiers and commands to be simplified. The 'dynamic' parameter
has been removed from BaseArgs (use isinstance instead).
Removal of the 'Static' prefix from CartesianProduct and Concatenate
The Args object is redundant with StaticArgs which has now replaced it
Renamed ListArgs to List and LinearArgs to Range
Setting an fp_precision of 0 specified numbers as ints (eg in Range)
Docstring fixes in core
Both Log and FilePattern now operate relative to getcwd

These classes used to use param.normalize_path but now they build
paths relative to os.get_cwd().  The _load_file_metadata hook has also
been removed from FilePattern.
The spec_formatter method is now a classmethod of BaseArgs
Parameter lacking constant=True and renamed listing variable to islist
Using unsortable_keys attribute to help exclude unsortable types
Reintroduced lexsorting as a lexsort method (more flexible than Pandas)
Renamed list_key to key and list_values to values in List class
Pretty-printing functionality now available as a mixin class
Introduced UnixCommand and eliminated a bug in CommandTemplate

The UnixCommand CommandTemplate offers a sufficiently general approach
for invoking commands on Unix that it is now offered as a concrete
example of how a new CommandTemplate may be implemented.
The timestamp format is now offered by the Launcher to CommandTemplates
Bug fix and set default response in review_and_launch to N