Ship a more sane .gitattributes with the project

This is related to the issues contributors have with the line endings.
This solution basically forbids line-ending changes on all files by
default and only marks specific files as "text" which then means that
the local settings of the person checking the files out will determine
what happens to the line endings on her machine.
Fixed issue in PWMsyn1ph: the equation "sigma = pre(sigma);" appeared to be used to cover the fact that sigma is only set during initialization, removed equation and changed sigma to a parameter with "fixed=false"
Set some parameters and constants to be declared as such for clarity
Removed use of non-standard function "classDirectory()" and replaced with modelica:// URL
Made partial model partial
Removed superfluous "import PowerSystems;" declarations
Replaced TableDir + "filename" with ModelicaServices.ExternalReferences.loadResource(TableDir + "filename") in order to handle URLs
Removed unspecified annotations "Window" and "DymolaStoredErrors"
Replaced __Dymola_checkBox with checkBox
Corrected hyperlinks in documentation
Removed recursive modification from component declaration
Replace isRoot() with Connections.isRoot()
Merge branch 'master' of https://github.com/harmanpa/PowerSystems