isregular() method for checking regularity of TS object

[chiraganand]

Methods

• isregular(ts::TS)
• isregular(ts::TS, unit::T) where {T<:Dates.Period}
• isregular(timestamps::T) where {T<:AbstractVector{TimeType}}
• isregular(timestamps::V, unit::T) {V<:AbstractVector{TimeType}, T<:Dates.Period}

Rules

1. The input is considered regular if the times are in a sequence that is strictly monotone (either increasing or decreasing) with a unique time step. REF: Matlab isregular()
2. For methods where the second argument <: TimeType is provided the input will be checked using rule 1 using the period provided.

Ref: #48

[chiraganand]

This was implemented in #130.