Addresses issue toDataFrame deprecation calls new to_pandas method but without returning the data frame. (see f6f7090)

Currently returns None, which causes e.g. tabular.to_csv() to fail.

Is this a bugfix or an enhancement?
bugfix
Proposed changes:

• return the output of to_pandas in toDataFrame shim
• use the non-deprecated, to_pandas call in output.py

Checklist:
The below is a list of things what will be considered when reviewing PRs. It is not prescriptive, and does not
imply that PRs which touch any of these will be rejected but gives a rough indication of where there is a potential
for hangups (i.e. factors which could turn a 5 min review into a half hour or longer and shunt it to the bottom
of the TODO list).

• Does the PR avoid variable renaming in existing code, whitespace changes, and other forms of tidying? [There is a place for code tidying, but it makes reviewing
  much simpler if this is kept separate from functional changes. The auto-formatting performed by some editors is particulaly egregious and can lead to files with thousands
  of non-functional changes with a few functional changes scattered amoungst them]

If an enhancement (or non-trivial bugfix):

• Has this been discussed in advance (feature request, PR proposal, email, or direct conversation)?
• Does this change how users interact with the software? How will these changes be communicated?
• Does this maintain backwards compatibility with old data?
• Does this change the required dependencies?
• Are there any other side effects of the change?