Improvements, Bugfixes, no more Windows 32-bit on Conda-Forge

A holiday release with bug fixes and improvements.

NOTE: Conda-Forge has dropped support for Windows 32-bit, so v0.2.7 is the last release that can be installed via conda on Windows 32-bit.

• FIXED Handle optional elements in ScalarVariable tag
• FIXED Handle null pointers in FMI logging
• FIXED Relax type of attribute "version" in fmiModelDescription.xsd to "normalizedString"
• FIXED Fix size of memory passed to fmi2DeSerializeFMUstate()
• NEW Validate modelDescription.outputs
• NEW Add compilation with Visual Studio 2017
• NEW Add missing attributes to ScalarVariable and validate derivatives and units
• NEW Change input extrapolation to "hold"