When excess curves defined in ~C, add null data and emit warning

[kinverarity1]

This PR fixes #83. Changes:

In LASFile.read, while we are assigning data to curves, we keep track of which curve indexes have had data assigned. Afterwards we iterate over all curve indices and for any that did not have data assigned we:

• emit a logger.warning message
• assign a data array full of np.nan values

[dcslagel]

Looks good. I'll go ahead and merge it.

[kinverarity1]

Thanks!