Fix Python extensions when compiled with clang on Linux 

When compiled with gcc the SingletonHolder instance symbols
are marked as globally unique (a GNU extension) but when compiled
with clang the symbols are marked as weak. Python extensions
are loaded by default without either RTLD_GLOBAL or RTLD_LOCAL
specified and the singleton symbols then remain private to each
module leading to multiple singleton instances within the same
process. This fix will also cover older versions of LD and is
a cleaner implementation than that removed in
afa941f0.

DPDReduction: error reading MomentumTransfer bins

Fix handling of runIndex in MergeMD

New unscripted testing page in developer docs

Fix missed GCC->GNU macro

Update CONTRIBUTING.md

Squashed commit of the following:

commit 7430d88e
Author: T Jubb <t.w.jubb@gmail.com>
Date:   Fri Aug 3 14:04:45 2018 +0100

    Refs #0 Revert PR template.

commit bfca2a5a
Merge: b7a9a09b ccc7a67d
Author: T Jubb <t.w.jubb@gmail.com>
Date:   Fri Aug 3 13:59:20 2018 +0100

    Refs #0 merge remote branch.

commit b7a9a09b
Merge: 3abde33d 16e27bdb
Author: T Jubb <t.w.jubb@gmail.com>
Date:   Fri Aug 3 13:55:47 2018 +0100

    Refs #0 merge master.

commit 3abde33d
Author: T Jubb <t.w.jubb@gmail.com>
Date:   Fri Aug 3 13:52:26 2018 +0100

    Refs #0 make changes as per comments.

commit a27f8a1a
Merge: 54684e10 b5ad845b
Author: T Jubb <t.w.jubb@gmail.com>
Date:   Fri Aug 3 13:51:18 2018 +0100

    Refs #0 merge master.

commit ccc7a67d
Merge: 54684e10 b5ad845b
Author: TWJubb <33163693+TWJubb@users.noreply.github.com>
Date:   Fri Aug 3 13:05:49 2018 +0100

    Merge branch 'master' into PyCharmDevDocs

commit 54684e10
Author: T Jubb <t.w.jubb@gmail.com>
Date:   Fri Aug 3 12:57:21 2018 +0100

    Refs #0 : Add PyCharm dev doc page.

Update third party and Fix sip build invocation

 Muon results table includes log values with value of 0

Fix clang warnings

Improve error handling for TableWorkspace

Minimal change to prevent a crash

Correct default doc string for empty property managers to dict()

SNAPReduce progress bar

Removed the LoadAscii Custom Dialog, it was out of date with the alg

Update MSlice to fix MakeProjection bug

Elemental Analysis: Peak Data Loading

Instrument definition file for DNS PSD detector

Fix detector rotations in TOFTOF IDF

Move time-weighted statistics to object

re #22807

RHEL7 devtoolset-7