Units.from_string

Parses a value and updates units with it

A Units expressed in string should match:

	units: wsp* unit-value wsp* unit-name? wsp*
	unit-value: number
	unit-name: 'px' | 'pt' | 'mm' | 'em' | 'cm'
	number: digit+
					| digit* sep digit+
	sep: '.' | ','
	digit: '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9'
	wsp: (#0x20 | #0x9 | #0xA | #0xB | #0xC | #0xD)+

For instance, these are valid strings:

	10 px
	5.1 em
	24 pt
	12.6 mm
	.3 cm

While these are not:

	42 cats
	omg!1!ponies

If no unit is specified, pixels are assumed.

Parameters:

str	the string to convert
units	a Units

Returns:

true if the string was successfully parsed, and false otherwise