add support for custom keywords with thermo output

doc/src/thermo_modify.rst

@@ -21,9 +21,14 @@ Syntax
        *norm* value = *yes* or *no*
        *flush* value = *yes* or *no*
        *line* value = *one* or *multi* or *yaml*
-       *format* values = *line* string, *int* string, *float* string, M string, or *none*
+       *header* values =  ID string, or *default*
+         string = new header name
+         ID = integer from 1 to N, or integer from -N to -1, where N = # of quantities being output
+              *or* a thermo keyword or reference to compute, fix, property or variable.
+       *format* values = *line* string, *int* string, *float* string, ID string, or *none*
          string = C-style format string
-         M = integer from 1 to N, where N = # of quantities being output
+         ID = integer from 1 to N, or integer from -N to -1, where N = # of quantities being output
+              *or* a thermo keyword or reference to compute, fix, property or variable.
        *temp* value = compute ID that calculates a temperature
        *press* value = compute ID that calculates a pressure
 
@@ -36,7 +41,8 @@ Examples
    thermo_modify temp myTemp format 3 %15.8g
    thermo_modify temp myTemp format line "%ld %g %g %15.8g"
    thermo_modify line multi format float %g
-   themos_modify line yaml format none
+   thermo_modify line yaml format none
+   thermo_modify header 1 Timestep header -2 Pressure header f_1[1] AvgDensity
 
 Description
 """""""""""
@@ -147,6 +153,20 @@ containing the timestep and CPU time ("multi"), or in a YAML format
 block ("yaml").  This modify option overrides the *one*, *multi*, or
 *yaml* thermo_style settings.
 
+The *header* keyword can be used to change the default header keyword
+for a column or field of thermodynamic output.  The setting for
+*ID string* replaces the default keyword with the provided string.
+*ID* can be a positive integer - then it represents the column number
+counting from the left -, a negative integer - then it represents the
+column number from the right (i.e. -1 is the last column/keyword),
+or a thermo keyword (or compute, fix, property, or variable reference)
+- then it replaces the string for that specific keyword -.
+
+The *header* keyword can be used multiple times. If multiple *header*
+settings refer to the same keyword, the last setting has precedence.
+the default setting is used.  A setting of *default* clears all previous
+settings, reverting all values to their default format.
+
 The *format* keyword can be used to change the default numeric format of
 any of quantities the :doc:`thermo_style <thermo_style>` command
 outputs.  All the specified format strings are C-style formats, e.g. as
@@ -155,12 +175,16 @@ argument which is the format string for the entire line of thermo
 output, with N fields, which you must enclose in quotes if it is more
 than one field.  The *int* and *float* keywords take a single format
 argument and are applied to all integer or floating-point quantities
-output.  The setting for *M string* also takes a single format argument
-which is used for the Mth value output in each line, e.g. the fifth
-column is output in high precision for "format 5 %20.15g".
+output.  The setting for *ID string* also takes a single format argument
+which is used for the indexed value in each line.  The interpretation is
+the same as for *header*, i.e. a positive integer is the n-th value corresponding
+to the n-th thermo keyword, a negative integer is counting backwards and
+a string matches the entry with the thermo keyword., e.g. the fifth
+column is output in high precision for "format 5 %20.15g" and the
+pair energy for "format epair %20.15g".
 
 The *format* keyword can be used multiple times.  The precedence is
-that for each value in a line of output, the *M* format (if specified)
+that for each value in a line of output, the *ID* format (if specified)
 is used, else the *int* or *float* setting (if specified) is used,
 else the *line* setting (if specified) for that value is used, else
 the default setting is used.  A setting of *none* clears all previous