Restructure NEON support

Currently NEON support is inconsistent. SIMDPP_USE_NEON corresponds to a
hypothetical architecture where both double and single precision
floating point arithmetic is done on a vector unit. Also,
SIMDPP_USE_NEON_VFP_DP has fewer features than SIMDPP_USE_NEON, which is
not intuitive.

This commit restructures the NEON support so that:
* SIMDPP_USE_NEON performs all floating-point computations on VFP
* SIMDPP_USE_NEON_FLT_SP performs single precision floating-point
computations on NEON.

Note, that currently NEON and VFP instructions are freely mixed, which
may lead to low performance.
convert: Do not use NEON for floating-point conversion operations

Floating-point conversion operations may be imprecise on NEON, thus
perform then on VFP when only USE_NEON is selected.