Add preprocess_coords_file

This function will handle any customization to the coordinates file that
needs to happen before this data gets plotted. Right now the only thing
that it does is to add the custom axes when requested.
Add option to use metadata categories as axes

The script interface has a new option to use metadata categories as
explicit axes in the plot.

This should close #6 as it is but there is still remaining work to be
able to name the axes properly.
Fix problem where unuseful samples wer not ignored

Samples in the mapping file but not in the coords file would make the
script fail with the --custom_axes option if the values in the metadata
mapping file was not numeric.
Add fill_mapping_field_from_mapping_file

This function will padd a metadata mapping file with given values in the
format of Category:ValueToPad;OtherCategory:OtherValueToPad.
Add --missing_custom_axes_values option

This option overrides the error raised when mapping file values are
passed with non-numeric values, the user is now capable of selecting
what this values are. Instead of removing the samples.
Add usage examples for --custom_axes

Modified some of the script interface help strings
Add action='append' to -x in the script interface

This change allows users to pass multiple category:value pairs without
using ';' as a delimiter.
Add comment documenting -x internal behaviour
Remove tab at the top of the file
Merge pull request #24 from ElDeveloper/issue_6

Custom axes -- Issue 6