Added EQUALREG json configuration
Added maskedSet() and initMask() GridProperties<T>
Added const to GridProperty::containsNan()
Added GridProperty::getDimensionString()
Added method GridProperties::getInitializedKeyword()

The getKeyword() method will autocreate and initialize a keyword if you
ask for one which is supported, but has not yet been accessed. The new
getInitializedkeyword() method will raise an exception if the keyword
has not already been initialized.
Added support for EQUALREG in EclipseState
Added test for EQUAL.
Added GridProperty::maskedMultiply() method
Added MULTIREG keyword.
Added support for MULTIREG keyword in EclipseState
Added GridProperty::maskedAdd() and maskedCopy()
Added ADDREG keyword
Added support for ADDREG keyword
Added COPYREG keyword
Added support for COPYREG keyword
Minor rebase fixup
Merge pull request #384 from joakim-hove/EQUALREG

Equalreg