vice.dataframe.filter
=====================

Obtain a copy of the dataframe whose elements satisfy a filter. Only
applies to dataframes whose values are all array-like.

**Signature**: x.filter(key, relation, value)

.. versionadded:: 1.1.0

Parameters
----------
x : ``dataframe``
	An instance of this class
key : ``str`` [case-insensitive]
	The dataframe key to filter based on
relation : ``str``
	Either '<', '<=', '=', '==', '!=', '>=', or '>', denoting the
	relation to filter based on.
value : real number
	The value to filter based on.

Returns
-------
filtered : ``dataframe``
	A dataframe whose elements are only those which satisfy the
	specified filter. This will always be an instance of the base
	class, even if the function called with an instance of a derived
	class.

Raises
------
* KeyError
	- Key is not in the dataframe
* ValueError
	- Invalid relation

Example Code
------------
>>> import vice
>>> example = vice.dataframe({
	"a": [1, 2, 3],
	"b": [4, 5, 6],
	"c": [7, 8, 9]})
>>> example
vice.dataframe{
	a --------------> [1, 2, 3]
	b --------------> [4, 5, 6]
	c --------------> [7, 8, 9]
}
>>> example.filter("a", "=", 2)
vice.dataframe{
	a --------------> [2]
	b --------------> [5]
	c --------------> [8]
}
>>> example.filter("c", "<=", 8)
vice.dataframe{
	a --------------> [1, 2]
	b --------------> [4, 5]
	c --------------> [7, 8]
}