Add heliiocentric coordinate transforms to SPEDAS

[jameswilburlewis]

From Rebecca Jolitz on SSL's Slack:

Question for Geo-Helio folks: is there an optimal way to do coordinate transforms from ECI/GEI to heliocentric coordinate systems (e.g. GSE, GSEQ) in IDL/SPEDAS?

We should add some heliocentric options to spd_cotrans.