Resolve "Double precision arithmetic in MDCT core"

• Related issues: #818 (closed)
• Requested reviewers: @fotopoulou

Reason why this change is needed

Replace double precisison arithmetic (sqrt()) by single precision pendant

Description of the change

Replace double precisison arithmetic (sqrt()) by single precision pendant

Affected operating points

• All operation points featuring kernel switching, i.e. high bitrate MDCT core; however, change is BE.
  • Produced bitstream is BE compared to previous state.
  • decoder output is BE compared to previous state.

Related to #818 (closed)