Gmos/mosaic

First implementation of the mosaic function. It currently takes a HDUList and a chipgap and makes an HDUList object. There's still a lot missing - specifically the image shift and transform that is performed by IRAF:

```
--> gmosaic gS20121010S0086.fits
GMOSAIC 
GMOSAIC -------------------------------------------------------------------
GMOSAIC Log opened at [Tue 19:30:41 30-Apr-2013]
GMOSAIC 
GMOSAIC --------------------
GMOSAIC Input Parameters:
GMOSAIC      inimages       = gS20121010S0086.fits
GMOSAIC      outimages      = 
GMOSAIC      outpref        = m
GMOSAIC      fl_paste       = no
GMOSAIC      fl_vardq       = no
GMOSAIC      fl_fixpix      = no
GMOSAIC      fl_clean       = yes
GMOSAIC      geointer       = linear
GMOSAIC      gap            = default
GMOSAIC      bpmfile        = gmos$data/chipgaps.dat
GMOSAIC      statsec        = default
GMOSAIC      obsmode        = IMAGE
GMOSAIC      sci_ext        = SCI
GMOSAIC      var_ext        = VAR
GMOSAIC      dq_ext         = DQ
GMOSAIC      key_detsec     = DETSEC
GMOSAIC      key_ccdsec     = CCDSEC
GMOSAIC      key_datsec     = DATASEC
GMOSAIC      key_ccdsum     = CCDSUM
GMOSAIC      key_obsmode    = OBSMODE
GMOSAIC      logfile        = 
GMOSAIC      verbose        = yes
GMOSAIC      fl_real      = no
GMOSAIC --------------------
GMOSAIC Input: gS20121010S0086  Output: mgS20121010S0086.fits
GMOSAIC Setting rotation Xrot[1]= 0.011000, Yrot[1]= 0.011000
GMOSAIC Setting rotation Xrot[2]= 0.000000, Yrot[2]= 0.000000
GMOSAIC Setting rotation Xrot[3]= 0.012000, Yrot[3]= 0.012000
GMOSAIC 
GMOSAIC Transforming image tmpimg21089_5[SCI,1][1:1024,1:2304] to image tmpchip21089_12
GMOSAIC     xshift: 1.02 yshift: 0.21 xmag: 1.00 ymag: 1.00 xrot: 0.01 yrot: 0.01
GMOSAIC 
GMOSAIC Transforming image tmpimg21089_5[SCI,2][1:1024,1:2304] to image tmpchip21089_13
GMOSAIC     xshift: 0.00 yshift: 0.00 xmag: 1.00 ymag: 1.00 xrot: 0.00 yrot: 0.00
GMOSAIC 
GMOSAIC Transforming image tmpimg21089_5[SCI,3][1:1024,1:2304] to image tmpchip21089_14
GMOSAIC     xshift: -2.90 yshift: -0.91 xmag: 1.00 ymag: 1.00 xrot: 0.01 yrot: 0.01
GMOSAIC imcopy  tmpchip21089_12  mgS20121010S0086.fits[1,append][1:1024,1:2304]  INDEF  0.
GMOSAIC imcopy  tmpchip21089_13  mgS20121010S0086.fits[1,append][1043:2066,1:2304]  INDEF  0.
GMOSAIC imcopy  tmpchip21089_14  mgS20121010S0086.fits[1,append][2085:3108,1:2304]  INDEF  0.
GMOSAIC All 1 images successfully mosaiced.
GMOSAIC --------------------
GMOSAIC Exit status: SUCCESS
GMOSAIC Log closed at [Tue 19:30:42 30-Apr-2013]
GMOSAIC -------------------------------------------------------------------
GMOSAIC 

```

**UPDATE** I added the mosaicing function now to GMOSPrepare with the exact same chipgap that `GMOSAIC` uses. This is calculated from defaults (specified in the yml file) and the binning factor.