Missing value management on the reference curve

[VelizarVESSELINOV]

If NULL is setup as 0, and the LAS file have reference DEPTH with value 0, today the library is replacing the value with nan for all the Curves, I know some other software packages that do this only for curves that are not reference.

LAS specification document is not very clear about how should be managed reference with NULL value, but I found it more logical to keep the reference without NULL replacement.

What do you think?

[kinverarity1]

I think your suggestion should be the default, with a keyword argument to control whether a substitution is made everywhere or not at all or with a different value to numpy.nan

[kinverarity1]

In light of #49 this could be a keyword argument index_null_replacement which is False by default.

[kinverarity1]

This issue is the same as #227 - which is now fixed!