add 2D slits to training instrument

[prjemian]

• close #89

[prjemian]

@cpchuang, @pken: This file adds the slits as slit1

[coveralls]

Pull Request Test Coverage Report for Build 4621253854

• 9 of 9 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 80.077%

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Totals
Change from base Build 4545648410:	0.2%
Covered Lines:	832
Relevant Lines:	1039

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💛 - Coveralls

[prjemian]

@rodolakis Thanks for the review. I made the changes you requested, then adjusted the documentation a bit more. Can you review again?

[prjemian]

Here is a full view of the most-recent edit: https://github.com/BCDA-APS/bluesky_training/blob/89-2d_slits/bluesky/instrument/devices/slits.py

[prjemian]

@rodolakis Thanks!