ENH : added q <-> d functions

 - reduce chances of messing this (simple) conversion up
TST : bumped up stringency an q <-> d round trip
ENH/TST : q <-> twotheta conversion functions

Connivance methods to prevent typo/confusion bugs
ENH/DOC : use convince methods from core

Use convenience methods from core for converting between q, d,
and two_theta.
MNT : change units of Si data to angstroms
ENH : added LaB6 standard data
MNT/API : tweaks to input parameters in calibration