Add support for writing .trc files

[halleysfifthinc]

writetrc(filename, f; <keyword arguments>)

Write the C3DFile f to a .trc format at filename.

Keyword arguments

• delim::Char='\t': The text delimiter to use
• strip_prefixes::Bool=true: Strip marker label prefixes if they exist
• subject::String="": The subject (among multiple subjects) in the C3DFile to write
• prefixes::Vector{String}=[subject]: Marker label prefixes to strip if
  strip_prefixes == true
• remove_unlabeled_markers::Bool=true: Remove markers with empty labels, labels
  matching r"(\*\d+|M\d\d\d)"
• lab_orientation::Matrix=Matrix(I, (3,3)): Rotation to apply to markers before writing
• precision::Int=6: Number of decimal places to print
• virtual_markers::Dict{String,Matrix}: (Optional) Virtual markers (calculated with markers
  from f) to write to the .trc file