Support case insensitive LinkType name in Feature creation

Note by @hkchekc on specifying LinkType in Feature creation after switching to Enums: https://github.com/G-Node/nixpy/pull/395#issuecomment-494399010

> @gicmo while the idea is great and being merged, I just want to note that the by-product (supporting old strings) is not going to work (at least not completely). In the past, the string 'Tagged' with the first letter capitalized works. Now the valid string parameter input should be all lower-case. For example, 'tagged'

Ah true, I guess for that a `.lower()` call would need to be added now. I have no strong opinion about supporting it (or not). @jgrewe what do you think?

_Originally posted by @gicmo in https://github.com/G-Node/nixpy/pull/395#issuecomment-494404537_

Start with the Feature creation path and the LinkType handling discussed in PR 395 and its linked comments. Confirm how string names are converted, then make the existing capitalized form such as "Tagged" work alongside the lowercase form and verify both inputs.

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