The templated constructor of GMPField fatched everything. We reduce the constructor
to those scalar values, which are convertible to mpf_class. As this removes the constructor
from char* we explicitly add constructor from char* and std::string.

This is the main part to fix FS#1665

Remove cast operator that caused Clang warnings.
This fixes FS#1207.

Kudos to Ansgar Burchardt for the patch.

[[Imported from SVN: r7064]]

With the exception of mpicollectivecommunication.hh
and propertymap.hh, which I didn't understand.

[[Imported from SVN: r6784]]

[[Imported from SVN: r5963]]

[[Imported from SVN: r5945]]

[[Imported from SVN: r5941]]

[[Imported from SVN: r5853]]