Core: add OVF file format support

[GPMueller]

http://math.nist.gov/oommf/doc/userguide11b2/userguide/vectorfieldformat.html

Related to #31.

[GPMueller]

There are also explanations at http://math.nist.gov/oommf/doc/userguide12b0/userguide.pdf

[GPMueller]

This may depend on #102 in order to load magnetization distributions of a different geometry.

[GPMueller]

Implemented first try with 13e0709, but it isn't working yet.
See here for a reference on the file format and example file.

[GPMueller]

Implemented binary OVF with PR #319, merged with 0b9db55.
However, mumax-generated OVF files could not be read. Fixed with 1f96358.

One should consider generalizing the file format to accept upper- and lower-case keywords alike by simply downcasing all the parsed strings.

TODO:

• the OVF text format cannot yet be read or written
• there are no proper tests about the number of cells and lattice type etc.

[GPMueller]

With 3f2eb1c on feature-refactor-geometry, the classification of the Geometry will be implemented, so that the compatibility of lattice types will be possible.

[GPMueller]

With PR #349, merged with cc5460d, the OVF file format has been completely implemented. Moved the remaining todo to issue #353.