Eigen integration

[mosra]

Done from both the coding and documentation side, except that it's failing to build on MSVC and Travis does not want to trigger the builds for latest commits.

TODO:

• [x] Convince Travis to build this, fix the remaining issues
• [x] Fix MSVC build
• [x] Clean up the dirty commits
• [x] Apply the following to Magnum post-merge:

diff --git a/doc/types.dox b/doc/types.dox
index d59abd9d9..677a2fd2a 100644
--- a/doc/types.dox
+++ b/doc/types.dox
@@ -216,6 +216,9 @@ documentation of each `Integration.h` header for details:

 -   Math-related Vulkan structures --- @ref Magnum/Vk/Integration.h, part of
     the @ref Vk library
+-   All Eigen types --- @ref Magnum/EigenIntegration/Integration.h and
+    @ref Magnum/EigenIntegration/GeometryIntegration.h, part of the
+    @ref EigenIntegration library
 -   All GLM types --- @ref Magnum/GlmIntegration/Integration.h,
     @ref Magnum/GlmIntegration/GtcIntegration.h and
     @ref Magnum/GlmIntegration/GtxIntegration.h, part of the

[codecov-io]

Codecov Report

Merging #40 into master will increase coverage by 2.73%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
+ Coverage      71%   73.73%   +2.73%     
==========================================
  Files          20       22       +2     
  Lines         769      849      +80     
==========================================
+ Hits          546      626      +80     
  Misses        223      223

Impacted Files	Coverage Δ
src/Magnum/EigenIntegration/GeometryIntegration.h	100% <100%> (ø)
src/Magnum/EigenIntegration/Integration.h	100% <100%> (ø)

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[mosra]

MSVC build fixed in 9f256844f688f9870f6ce6915f6ed96c01b2f514, fortunately it was enough to just specify the default template parameters explicitly. The rest was minor errors in Travis CI config, all green now.