Change usage to use list comprehension

Pd160 powgen cache func absorption calc

Allow and block options for LoadNexusLogs ornl-next

ADS return False when checking for None in python - ornl-next

Since lots of places want to check for
```
if value in mtd:
  ...
```
with randomly assigned values, change the interface to cleanly deal with
such things by checking if they are `None` then converting the value to
a string.

Remove mantidplot from ornl-next