Gmos/virtual slicing

added the slicing calculation and shift. Next step - streamline the process. Currently all revolves around the science_frame which is an abstract concept which has science fits, flat fits and arc fits links. It also contains the slices for all three data types. 

merge this after the science frame one.

```
In [1]: run -i read_raw_gemini_files.py
WARNING: ConfigurationDefaultMissingWarning: Requested default configuration file /home/wkerzend/python/geminiutil/geminiutil/geminiutil.cfg is not a file. Cannot install default profile. If you are importing from source, this is expected. [geminiutil]

In [2]: gmos.prepare_science_frame(proj.science[0].science_frame)

In [3]: test = proj.science[0].science_frame

In [4]: test.slices
Out[4]: 
[<GMOS MOS Slice (priority=0 lower_edge=741.76 upper_edge=755.46)>,
 <GMOS MOS Slice (priority=0 lower_edge=858.14 upper_edge=871.84)>,
 <GMOS MOS Slice (priority=0 lower_edge=1325.69 upper_edge=1339.39)>,
 <GMOS MOS Slice (priority=0 lower_edge=1474.08 upper_edge=1487.78)>,
 <GMOS MOS Slice (priority=3 lower_edge=64.35 upper_edge=98.60)>,
 <GMOS MOS Slice (priority=3 lower_edge=118.97 upper_edge=153.22)>,
 <GMOS MOS Slice (priority=3 lower_edge=176.86 upper_edge=211.11)>,
 <GMOS MOS Slice (priority=3 lower_edge=266.63 upper_edge=300.88)>,
 <GMOS MOS Slice (priority=3 lower_edge=220.79 upper_edge=255.03)>,
 <GMOS MOS Slice (priority=3 lower_edge=377.07 upper_edge=411.32)>,
 <GMOS MOS Slice (priority=3 lower_edge=415.71 upper_edge=449.96)>,
 <GMOS MOS Slice (priority=3 lower_edge=326.95 upper_edge=361.19)>,
 <GMOS MOS Slice (priority=3 lower_edge=514.98 upper_edge=549.23)>,
 <GMOS MOS Slice (priority=3 lower_edge=459.93 upper_edge=494.18)>,
 <GMOS MOS Slice (priority=3 lower_edge=601.83 upper_edge=636.08)>,
 <GMOS MOS Slice (priority=3 lower_edge=561.36 upper_edge=595.60)>,
 <GMOS MOS Slice (priority=3 lower_edge=666.36 upper_edge=700.61)>,
 <GMOS MOS Slice (priority=3 lower_edge=767.93 upper_edge=802.18)>,
 <GMOS MOS Slice (priority=1 lower_edge=1122.82 upper_edge=1163.58)>,
 <GMOS MOS Slice (priority=1 lower_edge=1088.84 upper_edge=1120.69)>,
 <GMOS MOS Slice (priority=1 lower_edge=1239.01 upper_edge=1269.83)>,
 <GMOS MOS Slice (priority=1 lower_edge=1207.47 upper_edge=1237.06)>,
 <GMOS MOS Slice (priority=1 lower_edge=1165.42 upper_edge=1205.28)>,
 <GMOS MOS Slice (priority=2 lower_edge=920.75 upper_edge=954.99)>,
 <GMOS MOS Slice (priority=2 lower_edge=1028.19 upper_edge=1062.44)>,
 <GMOS MOS Slice (priority=2 lower_edge=983.85 upper_edge=1018.10)>,
 <GMOS MOS Slice (priority=2 lower_edge=1273.54 upper_edge=1307.78)>,
 <GMOS MOS Slice (priority=2 lower_edge=1367.43 upper_edge=1401.68)>,
 <GMOS MOS Slice (priority=2 lower_edge=1410.44 upper_edge=1444.69)>,
 <GMOS MOS Slice (priority=3 lower_edge=1526.62 upper_edge=1560.86)>,
 <GMOS MOS Slice (priority=3 lower_edge=1565.68 upper_edge=1599.92)>,
 <GMOS MOS Slice (priority=3 lower_edge=1675.97 upper_edge=1710.22)>,
 <GMOS MOS Slice (priority=3 lower_edge=1635.77 upper_edge=1670.02)>,
 <GMOS MOS Slice (priority=3 lower_edge=1712.59 upper_edge=1746.84)>,
 <GMOS MOS Slice (priority=3 lower_edge=1817.33 upper_edge=1851.57)>,
 <GMOS MOS Slice (priority=3 lower_edge=1862.15 upper_edge=1896.40)>,
 <GMOS MOS Slice (priority=3 lower_edge=1776.74 upper_edge=1810.99)>,
 <GMOS MOS Slice (priority=3 lower_edge=1911.71 upper_edge=1945.95)>,
 <GMOS MOS Slice (priority=3 lower_edge=1948.31 upper_edge=1982.55)>,
 <GMOS MOS Slice (priority=3 lower_edge=2013.81 upper_edge=2048.06)>,
 <GMOS MOS Slice (priority=3 lower_edge=2085.25 upper_edge=2119.50)>,
 <GMOS MOS Slice (priority=3 lower_edge=2122.16 upper_edge=2156.40)>,
 <GMOS MOS Slice (priority=3 lower_edge=2203.47 upper_edge=2237.71)>,
 <GMOS MOS Slice (priority=3 lower_edge=2160.91 upper_edge=2195.15)>]
```