read OOMMF V2 files

[dehoni]

omv files created with OOMMF are shlightly different to what the GSC expects as input for magnetic files, e.g. the entry "valuemultiplier", "valuerangeminmag", "valuerangemaxmag" are optional, and "valueunit" can provide a value for each dimension.

The OMFReader should be able to handle the different cases.

[dehoni]

The entries "valuerangeminmag", "valuerangemaxmag" are actually never used later in the GCS. I commented them out for the moment. The other issues are handled such that the reader accepts files in the omf format seen so far.

[dehoni]

isolated_sk_T-Oxs_MinDriver-Magnetization-00-0000217.omf.txt
isolated_skyrmion.omf.txt
The two files are identical except of the header, which is OMF V2.0 for the first file and a V1 version called isolated_skyrmion.omf.

[dehoni]

The PR should provide a unit test, and a more meaningful message stating Version loaded. For failure the error message "We accept only Text format OMF file." is not specific enough, test if it is a omf file and where it fails (e.g. missing variable).