Change tests to run in parallel on travis.
Bump pandas version to 0.14
Change the PIP_WHEEL_COMMAND to the newer syntax.
Add a now required pandas dep for 2.6.

see this bug report: https://github.com/pydata/pandas/issues/7284
Merge pull request #1034 from Cadair/travis_parallel

[Review]: Make Travis Run Tests in Parallel
Fixed bugs in the RHESSI module, and updated the docstring to reflect the change of .show() to .peek()
Some tidying up of imports in RHESSI module.
Added back in the explicit import of sunpy.sun.constants
Merge pull request #1038 from transientlunatic/master

Updates to the RHESSI module.