Use robust units for deck input conversion factors

This PR makes the conversion factors of ConversionFactors.hpp more robust, and much easier to read by using proper units and prefixes in all places. To do that, this PR imports most of Units.hpp from opm-core into ConversionFactors.hpp (in the `details` namespace). While this duplication of code is unfortunate, it may be the least bad solution in the short term. Longer term solutions may be a unification of opm-parser and opm-core, or simply moving Units.hpp to opm-parser.

One important effect of this PR is to make input more consistent between opm-parser and the rest of opm, as well as with MRST. Without this, there is a relative discrepancy of approximately 1e-7 in pressure using field units for example.

Note: I seem to have a chain of extra merge commits that I do not why appeared. If they must be removed somehow, please inform me.