calc_indices: Calculate length based survey indices
In einarhjorleifsson/smx: MRI survey functions

Calculates abundance and biomass survey indices based on length classes for a particular species in a given year.

The function does in principle three things

1 2	calc_indices(st, le, lwcoeff = c(0.01, 3), stratas, std.towlength = 4, std.area = 4 * 17/1852, std.cv = 1)

`st`	A dataframe with station information. Required columns are id (unique station id), year, towlength and strata (the strata identifyer).
`le`	A dataframe with length frequency measurements. Required columns are id (station id), length (the length class) and n (the number of fish measured) where the latter are the "raised" numbers.
`lwcoeff`	A vector of length 2, containing parameter a and b of the length weight relationship.
`stratas,`	A dataframe containing columns strata (the strata identifyer) and area (the strata area).
`std.towlength`	Standard tow length in nautical miles.
`std.area`	Standardized area swept in nautical square miles.
`std.cv`	A multipler (default is 1) on the mean abundance/biomass if only one tow in a strata. In such cases the cv is set equivalent to the "mean" value.

Returns a list with the following data.frames:

base that contains the statistics by each strata
- year: Names/number of the strata
- ... TODO
aggr The total survey index. The columns are:
- year: Survey year
- length: The length class
- n: Abundance index for the given length class
- n.cv: cv of the abundance index for a given length class
- b: Biomass index for the given length class
- b.cv: cv of the biomass index for a given length class
- cn: Abundance index of fish smaller than or equal to a given length class.
- cn.cv: cv of the abundance index of fish smaller than or equal to a given length class
- cb: Biomass index of fish greater than or equal to a given length class.
- cb.cv: cv of the biomass index of fish greater than or equal to a given length class