dataframe_for_params
====================

A dataframe that directly serves parameters to objects, normally constructed as
part of a model object.  Should always have a Character column called parameter
and a Numeric column called value.  This is treated as a lookup table between
the parameter name and value.  If an object `obj` has a parameter name `par`, then
`obj::par` is examined to set the value; if no such name is in the table, then
`par` is examined to set the value; if no such name is in the table, then the
default internal to the object (type) is used.

set
===

	some_df set par value

Change the value of the parameter named `par` in `some_df` to be `value`.  Create
the row if it does not exist.  This will occur even if there is no object to 
receive it.

unset
=====

	some_df unset par

Remove the parameter named `par` from `some_df`.  This will occur even if it
results in an object using the default parameter without that parameter appearing
in the table.

details
=======

	some_df details

Emit the details of all parameters set and served by the system.  This will
include details of defaults not set in the table and of object parameters
set from the global parameter (i.e., all the `obj::par` instances will be listed
for each `par`).