FUNCTION get_dbl_pi
; Description: This function returns the value of pi as a double precision number. This function
; is necessary because the system variable "!PI" in IDL only contains pi as a single
; precision number (FLOAT type).
;
; Input Parameters:
;
; None.
;
; Return Value:
;
; The return value is a number of DOUBLE type whose value is equal to that of pi to 15 decimal
; places. The return value is also equal to the result of issuing the IDL command
; "x = acos(-1.0D)".
;
; Author: Dan Bramich (dan.bramich@hotmail.co.uk)
;
; History:
;
; 11/12/2011 - Module created (dmb).
;Return the value of pi as a number of double type which is precise to 15 decimal places