Reduce dependencies

[navidcy]

I removed PyPlot, Printf, and Coverage from main package deps and moved them in examples.

@glwagner, I think it's counterintuitive that people need to load, e.g., FourierFlows, FFTW, etc in a script, e.g., here:

GeophysicalFlows.jl/examples/twodturb_mcwilliams1984.jl

Lines 1 to 3 in 2311663

	using FourierFlows, PyPlot, JLD2, Printf, Random, FFTW
	using Random: seed!

Since these packages are deps of GeophysicalFlows.jl, I think that something like

using GeophysicalFlows, PyPlot

import GeophysicalFlows.TwoDTurb

should be the only thing we expect the users to do. What do you think?

[codecov]

Codecov Report

Merging #43 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #43   +/-   ##
=======================================
  Coverage   98.64%   98.64%           
=======================================
  Files           6        6           
  Lines         518      518           
=======================================
  Hits          511      511           
  Misses          7        7

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[coveralls]

Pull Request Test Coverage Report for Build 328

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 98.649%

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Totals
Change from base Build 324:	0.0%
Covered Lines:	1533
Relevant Lines:	1554

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💛 - Coveralls