ENH Plot multiple fixed arguments for detectors

[drewejohnson]

Summary of enhancement

Capabilities to plot several slices of detector data at once. Consider the example file bwr_det0.m. If one reads the whole file and obtains the xymesh detector with

import serpentTools
d = serpentTools.readDataFile('bwr_det0.m')['xymesh']

one can plot the tally data either with mesh plots or using the plot command, fixing the energy axis as

d.plot('xmesh', fixed={'energy': 0})

and obtain

There is a lot in this figure, and it is difficult to identify what lines correspond to what y-mesh plotted.

When completed, this enhancment will allow the user to plot along multiple axis, e.g. several y-mesh indexes, with one plot command.
By passing a tuple of values to the fixed dictionary for ymesh, one should be able to plot

Note the labels were added after the fact during my plot and are not indicative of the intended label. Instead, maybe add labels as the index, or allow the user to pass the labels through the labels argument.

The slice method does support this functionality, as demonstrated below

d.slice({'energy': 0, 'ymesh': (0, 4, 9)})
 [[8.19312e+17 7.18519e+17 6.90079e+17 6.22241e+17 5.97257e+17 5.97732e+17   5.23973e+17 5.18163e+17 4.47478e+17 4.42897e+17 4.16117e+17 4.57879e+17   4.83234e+17 5.27784e+17 5.48377e+17 5.90254e+17 6.06511e+17 6.77493e+17   8.22194e+17 8.80335e+17]
 [6.13839e+17 4.68601e+17 3.36648e+17 2.89372e+17 2.16846e+17 1.42831e+17   7.55625e+16 1.29467e+17 1.71221e+17 1.85019e+17 1.82318e+17 2.02328e+17   1.56334e+17 7.99477e+16 1.49016e+17 1.93624e+17 3.16721e+17 3.47858e+17   4.54319e+17 5.98720e+17]
 [4.53951e+17 3.73311e+17 2.59601e+17 2.11590e+17 2.07343e+17 1.82712e+17   1.98584e+17 2.38914e+17 3.17940e+17 3.76533e+17 4.43764e+17 3.72477e+17   3.25579e+17 1.87763e+17 1.68373e+17 1.88161e+17 2.14719e+17 2.60612e+17   3.42182e+17 4.72639e+17]]

Passing a similar dictionary into plot gets some errors with matplotlib, as the data is not the right shape.