PersistenceLandscape class in namespace versus exports

[corybrunson]

The package seems to install without issue, but i get an interesting warning when simulating installation:

> devtools::load_all()
ℹ Loading plt
Warning message:
Objects listed as exports, but not present in namespace:
• PersistenceLandscape 
> devtools::load_all()
ℹ Loading plt

Executing this code does not change the 'NAMESPACE' file; my guess is that it is somehow due to the order of the steps in that file, but that's based only on dissimilar past experience. I worry that the issue could pose confusing problems later if not addressed now.

[corybrunson]

@astamm i'd welcome your thoughts here.