Updated whatsnew with bugfixes for v1.7.3
Bump version to v1.7.3-rc.1
Merge pull request #1492 from dkillick/whatsnew_v1.7.3

Updated what's new doc with bugfixes for v1.7.3
Merge pull request #1493 from dkillick/release_candidate

Bump version to v1.7.3-rc.1
Added subsection of bugs fixed in v1.7.3
Merge pull request #1494 from dkillick/granularise_whatsnew

Subsection of bugs fixed in v1.7.3