Fix some of the slow down in tesscube, fix tuple problem.

[christinahedges]

@mrtommyb has pointed out that tesscube is slower than its previous versions. I have

1. Made some speed increases by calculating all the fits headers required on initialization of the TESSCube object, rather than during the TPF creation, as this is a one time calculation and can take ~1 second
2. Made the calculation of POS_CORR1 and POS_CORR2 optional. This step requires iterating through all the SPOC assigned WCS and calculating the source position. This is costly, but I've included it as this produces the most SPOC-like TPF product. It now has an optional flag.

[christinahedges]

Fixes #9 and Fixes #8

[christinahedges]

Closes #8 Closes #9