A non-exported function that checks whether compounding values frequencies are supported.
is_valid_compounding(compounding)a numeric vector representing the compounding frequency
a flag (TRUE or FALSE) if all the supplied compounding
frequencies are supported.
Valid compounding values are:
| Value | Frequency |
| -1 | Simply, T-bill discounting |
| 0 | Simply |
| 1 | Annually |
| 2 | Semi-annually |
| 3 | Tri-annually |
| 4 | Quarterly |
| 6 | Bi-monthly |
| 12 | Monthly |
| 24 | Fortnightly |
| 52 | Weekly |
| 365 | Daily |
| Inf | Continuously |