fixed SvgPath_getDimension() behaviour for multiple sub-paths

[redremus]

Solution for Issue #40
SvgPath_getDimension() now stores path origin on move ('m') and set abspos to it on path close ('z')

[saardrimer]

@threebytesfull has been working on this issue in a new version of PCBmodE. I have alerted him to this PR. I will not merge this PR to the current version as it will modify expected behaviour with existing designs. @threebytesfull will evaluate this PR against his version. Thanks!

[threebytesfull]

Ooh, that's a good spot - I hadn't found that one yet! :) I've got a load of test cases for the stuff I'd looked at so far in svgpath (tests at https://github.com/threebytesfull/pcbmode/blob/develop/tests/test_svgpath.py and corresponding code at https://github.com/threebytesfull/pcbmode/blob/develop/pcbmode/utils/svgpath.py - still work in progress!).

I had added tests for multi-step paths but not paths with close commands in between them - I'll add tests for that. I've made some fundamental changes which means my stuff can't be merged in without careful checking, hence the growing test suite. Want me to add tests and merge this PR into my develop branch?