MR Reads: Zero->NaN Padding

[ax3l]

Fill invalid field regions in MR with np.nan instead of zeros.

@RemiLehe on Slack:

matplotlib does handle np.nan gracefully:

• For colormaps, it just shows white color
• For line plots, it interrupts the curve at place where nan occurs.

[ax3l]

Added because they keep showing up in my git status

[RemiLehe]

This is working great:

• 2D plots with get_field and plot=True:
  

• 1D lineouts with get_field and plot=True:
  

[ax3l]

Yay!! 🤩
Thanks for the test!