Added option for lax vector conversions when compiling Helium code.

In relation with issue #58

When HELIUM, MVEF or MVEI are used as cmake options, the option
LAXVECTORCONVERSIONS can be set to ON or OFF to enable / disable
lax vector conversions in GCC or ArmClang.

Currently, CMSIS-DSP won't build if lax vector conversions are disabled.
The option is ON by default.