Convert <sp\> tag to a space

This caused significant problems with the parsing of comments around
code blocks.

Change-Id: I7bf498bed3fed65a71ec8f8bc81de3275f15b69b
Updating JSON doc string from ITK 4.6 with spacing corrections.

This includes spacing in code blocks and spaces round links and refs.

Change-Id: I945d03d8be98aebdd187ae41864c072b5dd13032
Merge topic 'SIMPLEITK-568_FixSpacingInDoxygenFromITK' into next

df6b0dc8 Updating JSON doc string from ITK 4.6 with spacing corrections.
c443435b Convert <sp\> tag to a space