Add tests and peek function for emission measure

[wtbarnes]

Related to comments in #110, this PR adds a method for quickly plotting the emission measure and also adds a few tests.

[codecov[bot]]

Codecov Report

Merging #113 into master will decrease coverage by 1.69%. The diff coverage is 14.81%.

@@            Coverage Diff             @@
##           master     #113      +/-   ##
==========================================
- Coverage   43.41%   41.71%   -1.70%     
==========================================
  Files          10       10              
  Lines         463      489      +26     
==========================================
+ Hits          201      204       +3     
- Misses        262      285      +23     

Impacted Files	Coverage Δ
pydrad/configure/configure.py	57.66% <ø> (ø)
pydrad/parse/parse.py	32.79% <14.28%> (-1.51%)	:arrow_down:
pydrad/visualize/plot.py	14.45% <15.38%> (-1.04%)	:arrow_down:

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[wtbarnes]

@jwreep with this PR, you should now be able to do something like,

import pydrad.parse
s = pydrad.parse.Strand('/path/to/my/hydrad')
s[10].peek_emission_measure()

to preview the column emission measure at the 10th timestep.