This commit is a manual replication of the contents of #2917, but targeting 3.0.x.

Do not convert contents of text JSON file to integers / floats then convert back to strings for storage in a KeyValues instance; instead just use the string representation. This fixes an issue whereby if a JSON is generated from an image, but that image also retains the same metadata fields in its own header, and then the JSON contents are re-imported, the two values for floating-point metadata elements could differ due to floating-point imprecision, and this would lead to the field being reset to the value "variable".

In order for a bvecs file to be properly read in, the information about how the image was transformed upon image load needs to be available. However if an attempt is made to perform such a read based on a Header that it has itself been constructed from an arbitrary ImageType, this information is lost. Therefore, reading from a bvecs file needs to be done using the Header class in which the corresponding NIfTI was originally opened.
In the process, some functionalities of the tracking of that transformation on image load have been made more explicit.
Also fixes some syntax errors relating to the manual back-porting of #2917 in #3011.

Concatenation of images along an axis other than 3, where phase encoding information differed, results in "PhaseEncodingDirection": "variable". Previously this was dissapearing silently; but with #3011, this instead threw an unhandled Exception.