ATC: fix_modify AtC on_the_fly

# fix_modify AtC on_the_fly

## syntax

fix_modify AtC on_the_fly <bond | kernel> <optional on | off>
- bond | kernel (keyword) = specifies on-the-fly calculation of bond or kernel matrix elements

- on | off (keyword) = activate or discontinue on-the-fly mode

## examples

`fix_modify AtC on_the_fly bond on`
`fix_modify AtC on_the_fly kernel`
`fix_modify AtC on_the_fly kernel off`

## description

Overrides normal mode of pre-calculating and storing bond pair-to-node a nd kernel atom-to-node matrices. If activated, will calculate elements of t hese matrices during repeated calls of field computations (i.e. "on-the-fly") and not store them for future use.
on flag is optional - if omitted, on_the_fly will be activated for the s pecified matrix. Can be deactivated using off flag.

## restrictions

Must be used with the hardy/field type of AtC fix ( see fix atc command )

## related

## default

By default, on-the-fly calculation is not active (i.e. off). However, code does a memory allocation check to determine if it can store all needed bond and kernel matrix ele ments. If this allocation fails, on-the-fly is activated.