add 48k filter design, small tweak to optimised scaling

[Allan-xmos]

Do you know roughly what the MIPS increase is for this over 16kHz?

As a guesstimate from the table here https://github.com/xmos/lib_mic_array/blob/develop/doc/programming_guide/src/resource_usage.rst :

• whole 16k 65 tap is 10.65 MIPS
• whole 32k 65 tap is 12.90 MIPS
• so just stage 2 16k 65 tap is 2.25 MIPS (12.90 - 10.65)
• stage 2 32k 65 tap is 5.50 MIPS
• stage 2 48k 65 tap is 7.75 MIPS -> whole 48k 65 tap is 15.65 MIPS
• stage 2 48k 96 tap is 11.63 MIPS -> whole 48k 96 tap is 20 MIPS

From that it looks like it might be worth trying a shorter stage 2 filter, so I'll make a "small" version as well